beat someone's ass

To beat someone's ass means to physically thrash or defeat them decisively — to win a fight so thoroughly there's no question about it. It's American vernacular at its most direct, used both to describe actual physical confrontations and as hyperbolic trash talk. You'll hear it in arguments, competitive sports smack talk, and dramatic retellings of altercations. It can also be used figuratively when one thing crushes another completely, like a team beating another team's ass on the scoreboard.

(US, idiomatic, vulgar) To thrash someone.