Someone asked ... what does the term "jackknife" means when referring to an accident? It is what happens when a vehicle pulling a trailer loses control in such a way that the trailer swings around and moves toward or even slams into the vehicle doing the hauling. Like the sample image shows, the truck kinda folds up life a jackknife. In a higher speed accident the trailer might smack into the cab. Or if other vehicles can become involved if they are hit by the out-of-control trailer. It isn't just truck drivers that have to worry about jackknifing. Cars or pick-ups hauling a trailer can also jackknife.