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.

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
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