A jackknife is a type of folding pocketknife where the blade folds into the handle, making it safe to carry in your pocket. The name comes from the way the blade pivots at one end, like a person bending sharply at the waist. Sailors and workers have carried jackknives for centuries because they're useful tools that don't take up much space.

The word also describes a dangerous accident involving vehicles with hinged parts. When a large truck jackknifes, its trailer swings sideways relative to the cab, often on slippery roads, forming a sharp angle like an opening pocketknife. The truck driver loses control as the two parts fold together at their connection point.

In diving, a jackknife dive is when you bend sharply at the waist mid-air, reaching toward your toes with straight legs, then straighten out before entering the water. The diver's body creates that same folding motion, bending and unbending in one smooth movement. It's one of the basic dives that competitive divers learn early in their training.