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.
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?
This is when a girl is getting fucked up the ass so hard that both her and the person fucking her knees buckle from the forces of fucking and getting fucked. Best observed in any porno where anal poundings are involved.
Dude me and my girl were fucking, it was Knee Buckling Butt Fucking night, she was so tired after that she fell asleep with my cock in her ass, what a night!
An insult, a derogitory remark usually slung in the face of the kind of trailer park trash the world would be better off without (more often used with a southern accent). It describes the actions of a male of questionable decent(possibly having been born from the womb of a female dog), having anal sex with a chosen partner(probaly a cousin)..