Green’s Dictionary of Slang

(US und.) a tool used to break through car door locks.

1978

(con. 1966) J. Carroll The Basketball Diaries 137: A ‘slapjack’ is a gizmo about as big as a very long screwdriver, with a weight on it that slides down after you insert the pin at the bottom into the lock on the car door. Bang! you slap down the weight toward the pin and it demolishes the fucking tumblers in the lock. Open door.