jack-knife

• jackknife (Note: the two spellings appear roughly equally common.)

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. (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. (statistics) Alternative spelling of jackknife

• clasp-knife
• flick knife
• penknife
• pocketknife, pocket knife
• Swiss Army knife
• switchblade

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.

   • 2009, Reif Larsen, The Selected Works of T.S. Spivet, Pinguin Books, p. 37:

     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.

A truck (lorry) jack-knifing

1. (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.