When you're fucking someone with such tenacity that they (and most likely you) lose the ability to formulate words or thoughts having to do with anything besides getting fucked harder, thus at the level of dumb BEASTS.
by Jack of Facts March 23, 2011
To have sexual intercourse with someone repugnant, usually in such a way as to avoid looking at them.
Tony: "Would you fuck her?"
Steve: "Hell no. She's a beast."
Tony: "Well, then beastfuck her."
Steve: ". . ."
Steve: "Hell no. She's a beast."
Tony: "Well, then beastfuck her."
Steve: ". . ."
by Big Steve July 01, 2004