Be flat on one’s back

the rear part of the human body, extending from the neck to the lower end of the spine.
the part of the body of animals corresponding to the human back.
the rear portion of any part of the body:
the back of the head.
the whole body, with reference to clothing:
the clothes on his back.
ability for labor; effort; endurance:
he put his back into the task.
the part opposite to or farthest from the front; the rear part:
the back of a hall.
the part that forms the rear of any object or structure:
the back of a chair.
the part that covers the back:
the back of a jacket.
the spine or backbone:
the fall broke his back.
any rear part of an object serving to support, protect, etc.:
the back of a binder.
nautical, aeronautics. the forward side of a propeller blade (opposed to face (def 20.)).
aeronautics. the top part or upper surface of an aircraft, especially of its fuselage.
bookbinding. the edge of a book formed where its sections are bound together.
the backs, grounds along the river cam in back of certain colleges at cambridge university in england: noted for their great beauty.
architecture, extrados.
carpentry.

the upper side of a joist, rafter, handrail, etc.
the area of interior wall between a window stool and the floor.

mining. the roof of a stope or drift.
sports.

a player whose regular position is behind that of players who make initial contact with the opposing team, as behind the forward line in football or nearest the player’s own goal in polo.
the position occupied by this player.

to support, as with authority, influence, help, or money (often followed by up):
to back a candidate; to back up a theory with facts.
to bet on:
to back a horse in the race.
to cause to move backward (often followed by up):
to back a car.
to furnish with a back:
to back a book.
to lie at the back of; form a back or background for:
a beach backed by hills.
to provide with an accompaniment:
a singer backed by piano and b-ss.
to get upon the back of; mount.
to write or print on the back of; endorse; countersign.
carpentry. to attach strips of wood to the upper edge of (a joist or rafter) to bring it to a desired level.
nautical.

to alter the position of (a sail) so that the wind will strike the forward face.
to brace (yards) in backing a sail.
to reinforce the hold of (an anchor) by means of a smaller one attached to it and dropped farther away.

to go or move backward (often followed by up).
nautical. (of wind) to change direction counterclockwise (opposed to veer).
situated at or in the rear:
at the back door; back fence.
far away or removed from the front or main area, position, or rank; remote:
back settlements.
belonging to the past:
back files; back issues.
in arrears; overdue:
back pay.
coming or going back; moving backward:
back current.
navigation, reciprocal (def 7).
phonetics. (of a speech sound) produced with the tongue articulating in the back part of the mouth, as in either of the sounds of go.
back away, to retreat; withdraw:
they gradually began to back away from their earlier opinion.
back down, to abandon an argument, opinion, or claim; withdraw; retreat:
he backed down as soon as a member of the audience challenged his -ssertion.
back off,

to back down:
now that the time for action had arrived, it was too late to back off.
textiles. to reverse (the spindle) in mule spinning prior to winding on the newly spun length of yarn.

back out (of), to fail to keep an engagement or promise; withdraw from; abandon:
two entrants have backed out of competing in the marathon. you can’t back out now.
back up,

to bring (a stream of traffic) to a standstill:
a stalled car backed up traffic for miles.
printing. to print a sheet again on its other side.
printing. to fill in (the thin copper sh-ll of an electrotype) with metal in order to strengthen it.
to move backward:
back up into the garage.
to reinforce:
we backed up the cardboard with slats so it wouldn’t fall down.
to support or confirm:
he backed up my story and they let us go.
computers. to duplicate (a file or a program) as a precaution against failure.

back up for, australian informal. to return for more of, as another helping of food.
back and fill,

nautical. to trim the sails of a boat so that the wind strikes them first on the forward and then on the after side.
to change one’s opinion or position; vacillate.

back and forth, south midland u.s.

to go back and forth, as in running errands or visiting:
he spent the day backing and forthing to the post office.
to work in an aimless or ineffective way; expend effort with little result.

back water,

nautical. to reverse the direction of a vessel.
to retreat from a position; withdraw an opinion:
i predict that the council will back water on the tax issue.

be flat on one’s back,

to be helpless or beaten:
he’s flat on his back after a long succession of failures.
to be confined to one’s bed because of illness.

behind one’s back, in one’s absence; without one’s knowledge; treacherously; secretly:
i’d rather talk to him about it directly than discuss it behind his back.
break someone’s back, to cause a person to fail, especially to cause to become bankrupt:
his family’s extravagance is breaking his back.
break the back of,

to complete the princ-p-l or hardest part of (a project, one’s work, etc.):
he finally broke the back of the problem.
to overcome; defeat:
they broke the back of our union.

get off one’s back, informal. to cease to find fault with or to disturb someone:
the fight started when they wouldn’t get off my back.
get one’s back up, informal. to become annoyed; take offense:
she gets her back up whenever someone mentions her family’s influence.
get / have / watch someone’s back, informal. to help and protect someone if necessary, especially in a time of trouble:
if he needs anything, i hope he knows i’ve got his back.
also, have got someone’s back.
have one’s back to the wall, to be in a difficult or hopeless situation.
in back of, behind:
he hid in back of the billboard. what could be in back of his strange behavior?
also, back of.
on one’s back, informal. finding fault with or disturbing someone:
the boss is always on my back about promptness.
pat on the back. pat1 (defs 9, 11).
a stab in the back. stab (def 12).
stab someone in the back. stab (def 13).
turn one’s back on,

to forsake or neglect:
he was unable to turn his back on any suffering creature.
to leave behind, as in anger.

noun
the posterior part of the human body, extending from the neck to the pelvis related adjective dorsal
the corresponding or upper part of an animal
the spinal column
the part or side of an object opposite the front
the part or side of anything less often seen or used: the back of a carpet, the back of a knife
the part or side of anything that is furthest from the front or from a spectator: the back of the stage
the convex part of something: the back of a hill, the back of a ship
something that supports, covers, or strengthens the rear of an object
(ball games)

a mainly defensive player behind a forward
the position of such a player

the part of a book to which the pages are glued or that joins the covers
(mining)

the side of a p-ssage or layer nearest the surface
the earth between that level and the next

the upper surface of a joist, rafter, slate, tile, etc, when in position compare bed (sense 13)
at one’s back, behind, esp in support or pursuit
at the back of one’s mind, not in one’s conscious thoughts
behind one’s back, without one’s knowledge; secretly or deceitfully
break one’s back, to overwork or work very hard
break the back of, to complete the greatest or hardest part of (a task)
on one’s back, flat on one’s back, incapacitated, esp through illness
(informal) get off someone’s back, to stop criticizing or pestering someone
have on one’s back, to be burdened with
(informal) on someone’s back, criticizing or pestering someone
put one’s back into, to devote all one’s strength to (a task)
put someone’s back up, get someone’s back up, to annoy someone
see the back of, to be rid of
back of beyond

the back of beyond, a very remote place
(austral) in such a place (esp in the phrase out back of beyond)

turn one’s back on

to turn away from in anger or contempt
to refuse to help; abandon

with one’s back to the wall, in a difficult or desperate situation
verb (mainly transitive)
(also intransitive) to move or cause to move backwards
to provide support, money, or encouragement for (a person, enterprise, etc)
to bet on the success of: to back a horse
to provide with a back, backing, or lining
to provide with a music accompaniment: a soloist backed by an orchestra
to provide a background for; be at the back of: mountains back the town
to countersign or endorse
(archaic) to mount the back of
(intransitive; foll by on or onto) to have the back facing (towards): the house backs onto a river
(intransitive) (of the wind) to change direction in an anticlockwise direction in the northern hemisphere and a clockwise direction in the southern see veer1 (sense 3a)
(nautical) to position (a sail) so that the wind presses on its opposite side
back and fill

(nautical) to manoeuvre the sails by alternately filling and emptying them of wind to navigate in a narrow place
to vacillate in one’s opinion

adjective (prenominal)
situated behind: a back lane
of the past: back issues of a magazine
owing from an earlier date: back rent
(mainly us & austral, nz) remote: back country
(of a road) not direct
moving in a backward direction: back current
(phonetics) of, relating to, or denoting a vowel articulated with the tongue retracted towards the soft palate, as for the vowels in english hard, fall, hot, full, fool
adverb
at, to, or towards the rear; away from something considered to be the front; backwards; behind
in, to, or towards the original starting point, place, or condition: to go back home, put the book back, my headache has come back
in or into the past: to look back on one’s childhood
in reply, repayment, or retaliation: to hit someone back, pay back a debt, to answer back
in check: the dam holds back the water
in concealment; in reserve: to keep something back, to hold back information
back and forth, to and fro
back to front

in reverse
in disorder

noun
a large tub or vat, esp one used by brewers
n.

old english bæc “back,” from proto-germanic -bakam (cf. old saxon and middle dutch bak, old frisian bek), with no known connections outside germanic.

the cognates mostly have been ousted in this sense in other modern germanic languages by words akin to modern english ridge (cf. danish ryg, german rücken). many indo-european languages show signs of once having distinguished the horizontal back of an animal (or a mountain range) from the upright back of a human. in other cases, a modern word for “back” may come from a word related to “spine” (italian schiena, russian spina) or “shoulder, shoulder blade” (spanish espalda, polish plecy).

to turn (one’s) back on (someone or something) “ignore” is from early 14c. behind (someone’s) back “clandestinely” is from late 14c.

to know (something) like the back of one’s hand, implying familiarity, is first attested 1893. the first attested use of the phrase is from a dismissive speech made to a character in robert louis stevenson’s “catriona”:

if i durst speak to herself, you may be certain i would never dream of trusting it to you; because i know you like the back of my hand, and all your bl-stering talk is that much wind to me.

the story, a sequel to “kidnapped,” has a scottish setting and context, and the back of my hand to you was noted in the late 19th century as a scottish expression meaning “i will have nothing to do with you” [e.g. “jamieson’s dictionary of the scottish language”]. in english generally, the back of (one’s) hand has been used to imply contempt and rejection since at least 1300. perhaps the connection of a menacing dismissal is what made stevenson choose that particular anatomical reference.

v.

late 15c., “to move (something) back,” from back (adv.). meaning “to support” (as by a bet) is first attested 1540s. related: backed; backing.
adj.

middle english, from back (n.) and back (adv.). formerly with comparative backer (c.1400), also backermore. to be on the back burner in the figurative sense is from 1960, from the image of a cook keeping a pot there to simmer while he or she works on another concoction at the front of the stove.
adv.

late 14c., shortened from abak, from old english on bæc “backwards, behind, aback” (see back (n.)). back and forth attested from 1814.

back (bāk)
n.

the posterior portion of the trunk of the human body between the neck and the pelvis; the dorsum.

the backbone or spine.

adverb

as a chaser: she wants whiskey with water back (1980s+)

noun

(also backup or backup for a beef) someone who will support and -ssist; a trusty ally (1980s+ teenagers)

verb

to give one’s support to some effort or person: i’ll back your application (1500s+)
to bet on: he backed green goo in the eighth (1600s+)
to contribute money for; bankroll: my cousin backed the rock show in the park (1880s+)

related terms

fishyback, get one’s or someone’s back up, get off someone’s back, get the monkey off, give someone the shirt off one’s back, knock back, laid-back, mellow-back, mossback, on someone’s back, piggyback, pin someone’s ears back, razorback, you scratch my back i scratch yours

back against the wall
back alley
back and fill
back and forth
back away
back burner, on a
back door
back down
back in circulation
back in harness
back number
back of
back of beyond
back of one’s hand
back of one’s mind
back off
back on one’s feet
back order
back out
back street
back the wrong horse
back to back
back to basics
back to the drawing board
back to the salt mines
back to the wall
back up
back water