The floorboards are weakest where they 'butt' (against each other). [=where they meet, where the ends of the boards touch each other]
As a child, she was a clumsy girl who was the 'butt of' everyone's jokes. [=people made jokes about her because she was clumsy]
Why don't you 'get off your butt' and do something? [=why don't you stop being so lazy and do something?]
(..) make you feel right at home. I know a couple of big old bull queers that'd just... ...love to make your acquaintance. Especially that big, white, mushy butt of yours. God! I don't belong here! -We have a winner! -I want to go home! And it's Fat Ass by a nose! (..)1994 The Shawshank Redemption
(..) and the fella she was banging. On the outside, he'd been vice president of a large Portland bank. Good work for a man so young. You speak English, butt-steak? You follow this officer. I never seen such a sorry-looking heap of maggot shit in all my life. Hey, fish! Come over here! (..)1994 The Shawshank Redemption