a large pocket knife with a folding blade

a dive in which the diver bends from the waist, touches the ankles without bending the knees, and straightens out before hitting the water

to cause (something) to double up like a jackknife

to double up like a jackknife

said of an articulated lorry: to go out of control so that the trailer swings round to form a V-shape and ends up at an angle of 90 degrees or less to the tractor

of a diver: to perform a jackknife