When a male inserts his penis AND testicles into the urethra of another male, causing the receiving male to have a "fat sack".
Guy 1: Honey,I want myself in you, but I'm tired of just anal
Guy 2:Okay, let's give me the fat sack.
Guy 2:Okay, let's give me the fat sack.
by Ballsballsballssack February 22, 2010