A derogatory term for a straight man who enjoys anal sex with other men. Usually used among friends in the case where one friend makes note of his bi-curiosity or that one-time homoerotic experience in college.
Guy 1: Did you hear that Kevin got a blow job by a trans guy at a party?
Guy 2: whaaaat?
Guy 1: yeah apparently he picked him up at a bar and thought it was a girl - kept going even when he found out it was a guy though. He was pretty drunk...
Guy 2: wow. What a butt boxer.