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[jak-nahyf] /ˈdʒækˌnaɪf/
noun, plural jackknives.
a large pocketknife.
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.
to bend or double over like a jackknife:
The prizefighter jackknifed and fell when he was hit in the stomach.
(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.
(in diving) to perform a jackknife.
to move rapidly at an abrupt angle.
verb (used with object), jackknifed, jackknifing.
to cause to jackknife:
The blow jackknifed the prizefighter.
to cut with a jackknife.
resembling a jackknife, as in its shape, function, or manner of opening and folding.
Origin of jackknife
1705-15, Americanism; jack1 (cf. jockteleg) + knife Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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Examples from the Web for jack-knife
Historical Examples
  • Kit bought one of these for a jack-knife,—for a curiosity, of course.

    Left on Labrador

    Charles Asbury Stephens
  • Quin, doubled up like a jack-knife beside her, was drunk with ecstasy.

    Quin Alice Hegan Rice
  • Yes, there was the cache I had made by splitting the pasteboard with my jack-knife.

    The Plum Tree David Graham Phillips
  • Then with his jack-knife he proceeded to investigate the inside.

    Tabitha's Vacation

    Ruth Alberta Brown
  • He had left his half-axe in camp, and when he felt in his pocket for his jack-knife it was not there.

    Forest Neighbors

    William Davenport Hulbert
  • Then the boy tuk his jack-knife 'n begun t' cut off the bear's tail.

    Eben Holden Irving Bacheller
  • Jim was kneeling on the ground, hacking with his jack-knife at the earth.

    Colorado Jim

    George Goodchild
  • Bet ye a jack-knife he'll be spreein' it fer all he's wuth to-morrow.

    The Panchronicon Harold Steele Mackaye
  • He pulled out his jack-knife and pushed it into the fog, clean to the handle.

    Jim Spurling, Fisherman

    Albert Walter Tolman
  • Cutter, with his jack-knife, parted the reins just back of the bit.

    The Octopus Frank Norris
British Dictionary definitions for jack-knife


noun (pl) -knives
a knife with the blade pivoted to fold into a recess in the handle
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)
(of an articulated lorry) to go out of control in such a way that the trailer swings round at an angle to the cab
to make a jackknife dive
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for jack-knife

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.


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.

Online Etymology Dictionary, © 2010 Douglas Harper
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