jack-knife

Short definition from Wiktionary

Noun

1. 1
   A compact folding knife.
2. 2
   The front-dive pike, in which the body folds and unfolds.
3. 3
   A semi-trailer truck accident in which the vehicle mimics the closing of a jack-knife.
4. 4
   Alternative spelling of jackknife.

Verb

1. 1
   To fold in the middle, as a jackknife does.
2. 2
   To cause a semi-trailer truck to fold like a jackknife in a traffic accident.