Word.ws English Dictionary

jack-knife

jack-knife noun: a compact folding knife. Also the alternative spelling of jackknife.

jack-knife: a compact folding knife
Image: Michael E. Cumpston, CC BY-SA 3.0, via Wikimedia Commons

Noun

jack-knife (plural jack-knives)

  1. A compact folding knife.
    • He kept a jack-knife in his pocket for various tasks.
  2. The front-dive pike, in which the body folds and unfolds.
    • It took me hundreds of dives to master even the simple jackknife.
  3. An abdominal exercise where knees and torso are raised from a lying position.
  4. (colloquial) A 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. A 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

Verb

jack-knife (third-person singular simple present jack-knifes, present participle jack-knifing, simple past and past participle jack-knifed)

  1. To fold in the middle, as a jackknife does.
    • The cat jackknifed in the air and landed gracefully on its feet.
    • 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
  2. (colloquial) To cause a semi-trailer truck to fold like a jackknife in a traffic accident.
    • Before I knew what was happening, I'd jack-knifed the truck.
    • Before I knew what was happening, I'd jack-knifed.

Also used as

jack-knife is also the alternative spelling of jackknife.

Synonyms of jack-knife

Forms of jack-knife

Noun
pluraljack-knives
alternative formjackknife
Verb
third-person singular presentjack-knifes
present participlejack-knifing
past participlejack-knifed
pastjack-knifed

Words formed from jack-knife

Phrases with jack-knife

Questions about jack-knife

What does jack-knife mean?
Jack-knife means a compact folding knife.
What part of speech is jack-knife?
Jack-knife can be a noun or a verb.
What is the plural of jack-knife?
The plural of jack-knife is jack-knives.

Words near jack-knife in the dictionary

  1. jack-in-the-pulpit
  2. jack-in-the-pulpits
  3. Jack-in-the-water
  4. Jack-jump-up-and-kiss-me
  5. Jack-jump-up-and-kiss-mes
  6. jack-knife
  7. jack-knife barber
  8. jack-knife barbers
  9. jack-knifed
  10. jack-knifes
  11. jack-knifing

Source and license

Adapted from Wiktionary: jack-knife (page history and authors). Text available under the Creative Commons Attribution-ShareAlike 4.0 license. Word.ws reformatted the entry and added links. Wiktionary data of 2026-09-02.

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