Jack-knife

[jak-nahyf] /ˈdʒækˌnaɪf/

noun, plural jackknives.
1.
a large pocketknife.
2.
Fancy Diving. a dive in which the diver bends in midair to touch the toes, keeping the legs straight, and then straightens out.
verb (used without object), jackknifed, jackknifing.
3.
to bend or double over like a jackknife:
The prizefighter jackknifed and fell when he was hit in the stomach.
4.
(of a trailer truck) to have the cab and trailer swivel at the linkage until they form a V shape, as the result of an abrupt stop or accident.
5.
(in diving) to perform a jackknife.
6.
to move rapidly at an abrupt angle.
verb (used with object), jackknifed, jackknifing.
7.
to cause to jackknife:
The blow jackknifed the prizefighter.
8.
to cut with a jackknife.
adjective
9.
resembling a jackknife, as in its shape, function, or manner of opening and folding.
/ˈdʒækˌnaɪf/
noun (pl) -knives
1.
a knife with the blade pivoted to fold into a recess in the handle
2.
a former name for a type of dive in which the diver bends at the waist in midair, with his legs straight and his hands touching his feet, finally straightening out and entering the water headfirst: forward pike dive
verb (intransitive)
3.
(of an articulated lorry) to go out of control in such a way that the trailer swings round at an angle to the cab
4.
to make a jackknife dive
n.

also jackknife, large pocket knife, 1711, probably American English, “perh[aps] associated with some sense of JACK sb.1, but cf. jackleg knife” [OED]; see jack + knife (n.). Jackleg was a U.S. colloquial term of contempt from c.1850. On another theory, so called because it originally was associated with sailors. As a kind of swimming dive, from 1922. As a type of tractor-trailer accident, 1966. Both from the notion of folding, as the knife does.
v.

1776, “to stab,” from jack-knife (n.). Intransitive meaning “to fold or bend” the body is said to date from the time of the American Civil War. The truck accident verbal sense is from 1949. Related: Jackknifed; jackknifing.