when you stick a penis (or dildo) up someones ass for your sexual pleasure. this happens a lot with queers. but a man could stick his weiner in a womens ass or a women could stick a dildo up another mans ass. there are endless possibilities
me and your wife were butt fucking last night and thats why theres a white spot in her ass.
A term used to describe an incredibly ugly person; someone you'd have to turn around to fuck. For people who aren't into anal, this is somewhat drastic.
Can also be shortened to "butt fugly".
Robin: Dude, look at the chick! She's so butt-fucking ugly!
Oliver: I dunno, man, I think I'd just paper bag her.
James: Hey, fuck off, dude! That's my sister!
Oliver: Family resemblance, eh?