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 "
by Capel & Droughton November 19, 2011

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