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.
When in the course of your day you took a big dump but was unable to get your ass crack as clean as it needed to be because of those lame ass little toilet paper squares in the high school bathroom or the local gas station.
Stinkbutt usually comes into play when it inhibits people from having sex until they have had a proper bath.
I wanted to make it with the chick but I knew I had a bad case of stinkbutt.