A person's ass that is full of pimples and/or pockmarks; similar to how kids with acne were called "pizza`face" back in grade school, except this term is specific to the buttock area.
Matt: "Alex mooned me yesterday and I was like, 'damn! I didn't order no pepperoni & butthole pizza!'"
Cara: "Ha! I called him up and ordered it for you and he said he'd deliver for free."
Matt: "Well, there was way too much pepperoni for my liking."
Cara: "Yeah, and probably not enough butthole, huh? ha!"