A man or a woman, who is submissive by nature. The buttboy (unisex) will always be the weaker individual in a relationship. The buttboy will bend over and present themselves to the dom regularly, because a buttboy lives to be a spineless individual. Anyone who is a whiner, or an overly sensitive person can be considered a buttboy as well, it is not necessarily sexual.
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi