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