Bwahahahahahahahahahahaha!!!! Okay, I know it ain't funny, but WTF is "monkey butt"? Sorry, I have not had the privilege to serve.
It's a rash you get on the lower portion of your cheeks. If you look at it in a mirror, it looks like a monkey's butt.
Motocrossers, bicyclists, desert racers, etc., tend to get an occasional case of it from time to time.
Baby powder and diaper rash creme help (A LOT!)....
