Definition of "jack-knife" in English

noun

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. (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.

4. 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; mumblety-peg.

5. (statistics) Alternative spelling of jackknife.

verb

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

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

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.