Butt nipple

The term used for a very large, painful, and uncomfortable pimple located on your gluteus maximus, or more widely named, your butt.

Guy 1: "Hey man, you down for a couple drinks at the pub tonight?"
Guy 2: "Sorry man, I can't. I got a huge butt nipple my wife's supposed to take care of for me."
Guy 1: "Ah man, I understand. Butt nipples are a bitch."