Someone who wears trousers that hang too tightly around the arse. The trousers are usually so tight, that it appears the the persons butt is actually munching on their trousers. It is not a pretty sight.
Related illnesses: Ankle Swingers, which often accompanies Butt Munching, or the type of person who it prone to Butt Munching.
"Oh my god, see that guys trousers!"
"Euw, Butt Munch"
"Those cheeks are going to start talking to us in a moment!"