Other spellings

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Singular
jack-knife

Plural
jack-knives

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

Plain form
jack-knife

Third-person singular
jack-knifes

Past tense
jack-knifed

Past participle
jack-knifed

Present participle
jack-knifing

 
A truck (lorry) jack-knifing.
  1. (intransitive) If something jack-knifes, it folds in the middle like a jack-knife.
  2. (intransitive) (informal) If a a semi-trailer truck jack-knifes, it folds like a jack-knife in a traffic accident.