n. A large clasp knife.n. Sports A dive in the pike position, in which the diver straightens out to enter the water hands first.v. To fold or double (something or oneself) like a jackknife.v. To cut or stab with a jackknife.verb-intransitive. To bend or fold up like a jackknife: A truck that had jackknifed was blocking the road. verb-intransitive. To form a 90° angle.