Definitions

1. verb

past tense of jackknife: to turn the front of a vehicle, especially a truck or trailer, sideways, often because of a loss of control or a collision

“The truck jackknifed on the icy road, causing a massive pileup.”

2. noun

a situation in which a vehicle, especially a truck or trailer, turns sideways, often because of a loss of control or a collision

“The jackknifed truck blocked the entire highway, causing traffic to back up for miles.”