<p><strong>Noun</strong></p><ul><li>a large pocketknife.</li><li>a dive in which the diver bends in midair to touch the toes, keeping the legs straight, and then straightens out.</li></ul><p><strong>Verb</strong> (used without object)</p><ul><li>to bend or double over like a jackknife.</li><li>(of a trailer truck) to have the cab and trailer swivel at the linkage until they form a V shape, as the result of an abrupt stop or accident.</li><li>(in diving) to perform a jackknife.</li><li>to move rapidly at an abrupt angle.</li></ul><p><strong>Verb</strong> (used with object)</p><ul><li>to cause to jackknife.</li><li>to cut with a jackknife.</li></ul><p><strong>Adjective</strong></p><ul><li>resembling a jackknife, as in its shape, function, or manner of opening and folding.</li></ul>