the act of thrusting the hips back and forth while naked causing the genitals to swing like a pendulum alternately slapping the stomach and butt.

Usually performed by males though some females with ample piss flaps are capable of this exercise while some males regrettably, are incapable.
Mark broke the ice when he did the stomach to butt and got the party started.

I was shocked when Mary's mother did the stomach to butt at the wedding reception , her wizard sleeve looked like balls swinging.
by theanarchist July 1, 2013
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When your stomach is so pudgie that the bottom part of your stomach forms a lower case "w" shape making it look like a butt.
"Ew, look how gross that girl's stomach butt is."

"She's so pudgie she has a stomach butt"
by Mellow Jello January 8, 2014
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