Sack Flapping

The sound your ballsack makes when pounding a tight slizz from behind; a good sack flap goes to a beat and makes a soft thumping noise... & can lead to minor bruising and irritation of the stomach region of the partner whos tight slizz or smelly balloon knot is being ravaged

Kelly: "Whats that on your stomach?"
Sara: "I had a good Sack flapping last night"
Kelly: "Oh i got sack flapped last wednesday; and his beanbag bruised the hell out of my tummy "