jack-knife

plural jack-knives · alternative jackknife

1. A compact folding knife.

   He kept a jack-knife in his pocket for various tasks.

2. divingThe front-dive pike, in which the body folds and unfolds.

   It took me hundreds of dives to master even the simple jackknife.

3. hobbiesAn abdominal exercise where knees and torso are raised from a lying position.
4. colloquialroadA semi-trailer truck accident in which the vehicle mimics the closing of a jack-knife.

   I have seen several jack-knives along that dangerous stretch of road.

5. gamesA game of skill played with a pocket knife in which participants try to throw or flip the knife to stick upright in the ground at specified points or areas.

   Over at the stable, the men joke and drink whiskey, talk crops and play jack-knife […]

   1948, Truman Capote, chapter 1, in Other Voices, Other Rooms, Part One

   Synonyms: mumblety peg

6. alternativemathematicsAlternative spelling of jackknife.

present singular third person jack-knifes · participle present jack-knifing · participle past jack-knifed · past jack-knifed · alternative jackknife

1. To fold in the middle, as a jackknife does.

   The cat jackknifed in the air and landed gracefully on its feet.

   2 more examples

   There was a world inside that tall grass. You could plop yourself down in the middle of it with the scraggly stems against the back of your neck and the endless grasses rising up and jackknifing against the bigbluesky, and the ranch and all of its players would fade into a distant dream.

   2009, Reif Larsen, The Selected Works of T.S. Spivet, Pinguin Books, page 37

   Ten of the freight train's wagons had derailed, two of them foul of the Down Fast. At 2308, the TPO struck one of them, causing the locomotive (86239) to jack-knife and turn so that it was almost touching the end wall of a house in The Russetts, a new-build row that sat close to the line.

   2026 March 18, Greg Morse, “1996 crash driving freight reforms”, in RAIL, number 1057, page 50

2. colloquialTo cause a semi-trailer truck to fold like a jackknife in a traffic accident.

   Before I knew what was happening, I'd jack-knifed.

   1 more example

   Before I knew what was happening, I'd jack-knifed the truck.