When a person accidentally poops while mooning someone, and the smell of their own feces causes them to make the stank face.
Cheryl had diarrhea, but had the sudden urge to moon Josh. While mooning Josh, Cheryl started to release her feces, and the smell caused her to express the stank face. And Josh experienced the full effects of the StankButt.
by PRUSUS June 30, 2014
Get the StankButtmug. 
Get the stankbuttermug.