—n. 1. a large pocketknife.
2. Fancy Diving.a dive in which the diver bends in midair to touch the toes, keeping the legs straight, and then straightens out.
—v.i. 1. to bend or double over like a jackknife: The prizefighter jackknifed and fell when he was hit in the stomach. 2. (of a trailer truck) to have the cab and trailer swivel at the linkage until they form a Vshape, as the result of an abrupt stop or accident.
3. (in diving) to perform a jackknife.
4. to move rapidly at an abrupt angle.
—v.t. 1. to cause to jackknife: The blow jackknifed the prizefighter. 2. to cut with a jackknife.
—adj. resembling a jackknife, as in its shape, function, or manner of opening and folding.