jack·knife 
(jăknīf′)
(jăknīf′)n.
1. A large clasp knife.
2. Sports A dive in the pike position, in which the diver straightens out to enter the water hands first.
v. jack·knifed, jack·knif·ing, jack·knifes
v.tr.
1. To fold or double (something or oneself) like a jackknife.
2. To cut or stab with a jackknife.
v.intr.
1. To bend or fold up like a jackknife: A truck that had jackknifed was blocking the road.
2. To form a 90° angle.