Jackknifing

Part of speech: verb

Definitions

1. The action of bending or folding sharply, especially used to describe a vehicle folding at the hinge point during sudden braking or impact
2. A sudden sharp bending, commonly referring to the folding of a truck's trailer toward the cab in a crash or emergency stop
3. When something, notably a vehicle, folds abruptly at a joint, often seen in accidents where the back section swings toward the front

Etymology: "Jackknifing" is the present participle of "jackknife," so its real story belongs to the base verb. A jackknife was originally a large folding pocket knife, and the verb took on the sense of a long, hinged object folding sharply back on itself, much as the blade snaps shut against its handle. The image is specific: the knife's blade folds into a sharp angle, and that is exactly how a tractor-trailer behaves when its cab swings sharply against the trailer at the hinge point. Used of vehicles, the word describes a trailer skidding sideways until it swings around at roughly a right angle to the tractor, the two units folding together like the closing blade. Because the trailer usually breaks free from the braking tractor, the term is most often used of a sudden loss of control on a wet or icy road. The same folding image also explains its use for the diving position in which a swimmer bends sharply at the waist and touches the toes.

Synonyms: folding, bending