Other spellings

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Singular
jackknife

Plural
jackknives

 
Utility jackknives.
 
A jackknife dive.
 
A jackknifed truck.
  1. (countable) A jackknife is a compact folding knife.
  2. (countable) (diving) A jackknife is a front-dive pike where the body folds and unfolds.
  3. (countable) (informal) A jackknife is a semi-trailer truck accident where the vehicle mimics the closing of a jackknife.

Plain form
jackknife

Third-person singular
jackknifes

Past tense
jackknifed

Past participle
jackknifed

Present participle
jackknifing

 
A truck (lorry) jackknifing.
  1. (intransitive) If something jackknifes, it folds in the middle like a jackknife.
  2. (intransitive) (informal) If a a semi-trailer truck jackknifes, it folds like a jackknife in a traffic accident.