godan verb (-su)
Colloquial and somewhat vulgar; used among young people or in casual contexts. The verb はめる (to insert/fit) combined with 倒す (to do thoroughly) gives a sense of doing it intensely or completely.
昨日、彼女とはめ倒した。
I had sex with my girlfriend yesterday.
はめる alone can mean 'to have sex' in slang, but はめ倒す intensifies the action, implying doing it vigorously or to completion.
Compound of はめる (to insert, fit; slang for 'to have sex') and 倒す (to do something thoroughly or completely). The exact origin is uncertain, but it likely emerged as a colloquial intensifier.