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1.
The backhanding of a man's genatalia causing extreme pain and can cause him to fall down and cry like a 6 year old girl.

A sac whack can be performed mid way through a conversation to maximise laughter and unexpected pain.
Eg:

Bob: "Hey man wanna go down to Smoky Jim's and get a bur-"
Steve: "SAC WHACK!" *backhands bob in groin*
by Dick Cheese June 24, 2005