What does jackknife mean?
A jackknife is a folding knife whose blade folds into the handle, making it compact and easy to carry. The term can also refer, in maritime language, to a triangular sail set square to the wind. As a verb, jackknife means to cut or slash with a knife, while in nautical contexts it can describe a vessel turning suddenly so that its sails become square to the wind. The knife sense is everyday and is especially familiar in North American English, where jackknife is a common general term for a pocket or folding knife. The word is useful because it links a familiar practical object with a less familiar but traditional maritime meaning. It is also distinct from a penknife, which is specifically a small, typically pen-sized folding knife.
A folding knife with a blade that pivots into the handle; in nautical use, a triangular sail set square to the wind.
"She used the jackknife to open the package."
To cut or slash with a jackknife; in nautical use, to turn a vessel suddenly so that its sails are square to the wind.
"The wind shifted and the boat jackknifed in the choppy water."
The usual plural is jackknives, with the final e changing to -es.
"He carried several jackknives in his tool belt."
A jackknife can fold into its own handle, while sailors use the same word for a sail that is set square to the wind.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does jackknife come from?
The word is a compound of jack and knife, although the precise historical reason for the element jack is uncertain. Its nautical senses developed from the idea of bringing a vessel's sails square to the wind, and the verb later extended to describe a sudden turn in that direction. The knife meaning is older and refers to a folding blade with the handle functioning as a sheath.
How jackknife is actually used
As a noun for a folding knife, jackknife is especially common in North American English; in British English, clasp knife is often preferred. The nautical senses are specialized and are used mainly in maritime contexts.
Easily confused with jackknife
A penknife is a small folding knife designed to fit in a pocket, while a jackknife is a broader, more general term for a folding knife and also has a distinct nautical meaning.