See more synonyms for jackknife on Thesaurus.com
noun, plural jack·knives.
  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), jack·knifed, jack·knif·ing.
  1. to bend or double over like a jackknife: The prizefighter jackknifed and fell when he was hit in the stomach.
  2. (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.
  3. (in diving) to perform a jackknife.
  4. to move rapidly at an abrupt angle.
verb (used with object), jack·knifed, jack·knif·ing.
  1. to cause to jackknife: The blow jackknifed the prizefighter.
  2. to cut with a jackknife.
  1. resembling a jackknife, as in its shape, function, or manner of opening and folding.

Origin of jackknife

1705–15, Americanism; jack1 (cf. jockteleg) + knife
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words for jackknife

plunge, cannonball, gainer, jackknife

Examples from the Web for jackknife

Historical Examples of jackknife

  • Robert drew a jackknife from his pocket, and did as he was bidden.

    Brave and Bold

    Horatio Alger

  • The black strips at the end of each nail, Martin pared off with his jackknife.


    Mr. and Mrs. Haldeman-Julius

  • He took out his jackknife, and reaching over, severed the traces.

    Cap'n Eri

    Joseph Crosby Lincoln

  • Then he pulled out his jackknife and felt its sharp edge with his thumb.

    The Tale of Timothy Turtle

    Arthur Scott Bailey

  • He carried a whetstone, on which he was sharpening his jackknife.

    Little Prudy

    Sophie May

British Dictionary definitions for jackknife


noun plural -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 (intr)
  1. (of an articulated lorry) to go out of control in such a way that the trailer swings round at an angle to the cab
  2. 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