In a internal combustion engine when the cylinder is oversized and the piston will rock side to side producing a knocking sound.
Chad's car has really bad pistion slap
af Nobis 29. juli 2003
1 more definition
The act of fuckin a loose hoe, whose pussy is so loose a man's crank will slap around in it.
When I was fuckin Cathy i suffered from piston slap.
af Paul Trahan 21. april 2003