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When a male is bored in the shower and stretches his scrotum out and bounces his penis off of it so that looks like his penis is bouncing on a trampoline.
Guy 1. "You know when you're bored in the shower and you stretch your ballsack out and bounce your dick off of it like it's bouncing on a trampoline?"

Guy 2. "What the fuck??? No...."

Guy 3. "Oh, you mean playing the sack drum?"

Guy 1. "Yeah, exactly."
by WowDad August 05, 2011