At the ready

completely prepared or in fit condition for immediate action or use:
troops ready for battle; Dinner is ready.
duly equipped, completed, adjusted, or arranged, as for an occasion or purpose:
The mechanic called to say that the car is ready.
willing:
ready to forgive.
prompt or quick in perceiving, comprehending, speaking, writing, etc.
proceeding from or showing such quickness:
a ready reply.
prompt or quick in action, performance, manifestation, etc.:
a keen mind and ready wit.
inclined; disposed; apt:
too ready to criticize others.
in such a condition as to be imminent; likely at any moment:
a tree ready to fall.
immediately available for use:
a ready source of cash.
pertaining to prompt payment.
present or convenient:
to lie ready to one’s hand.
to make ready; prepare.
the state or condition of being ready.
Informal. ; cash.
(used in calling the start of a race to indicate that racers should be prepared to start):
Ready! Set! Go!
at the ready, in a condition of readiness, available for immediate use:
shoppers with their umbrellas at the ready; soldiers keeping their weapons at the ready.
get ready!, (in calling the start of a race) be prepared to start:
Get ready! Get set! Go!
make ready,

to bring to a state of readiness or completion; prepare.
Printing. to ready a press for printing.

ready up, British and Australian Slang. to swindle.
adjective readier, readiest
in a state of completion or preparedness, as for use or action
willing or eager: ready helpers
prompt or rapid: a ready response
(prenominal) quick in perceiving; intelligent: a ready mind
(postpositive) foll by to. on the point (of) or liable (to): ready to collapse
(postpositive) conveniently near (esp in the phrase ready to hand)
make ready, get ready, to prepare oneself or something for use or action
noun
(informal) the ready, short for ready money
at the ready

poised for use or action: with pen at the ready
(of a rifle) in the position normally adopted immediately prior to aiming and firing

verb
(transitive) to put in a state of readiness; prepare
adj.

Old English ræde, geræde “prepared, ready,” of a horse, “ready for riding,” from Proto-Germanic *garaidijaz “arranged” (cf. Old Frisian rede “ready,” Middle Dutch gereit, Old High German reiti, Middle High German bereite, German bereit, Old Norse greiðr “ready, plain,” Gothic garaiþs “ordered, arranged”), from PIE root *reidh- “to ride” (see ride (v.)). Lengthened in Middle English by change of ending. Ready-made first attested early 15c.; ready-to-wear is from 1890.
v.

early 13c., “to administer;” c.1300, “to take aim;” mid-14c., “to prepare, make ready,” from ready (adj.). Related: Readied; readying.
Available for immediate use, as in Umbrellas at the ready, we were prepared to brave the storm. This idiom was originally a military term in which the ready denoted the position of a firearm prepared to be raised and aimed or fired. [ First half of 1800s ]
In addition to the idiom beginning with
ready
also see:

at the ready
get ready
good and (ready)
rough and ready