I. ˈjack-knife 1 BrE AmE noun ( plural jack-knives ) [countable]
1 . a knife with a blade that folds into its handle
2 . a ↑ dive in which you bend at the waist when you are in the air
II. jack-knife 2 BrE AmE verb [intransitive]
if a large vehicle with two parts jack-knifes, it slides out of control and the back part swings towards the front part:
The truck skidded on the ice and jack-knifed.