Used in a sentence typically constructed as "You're making an ass out of yourself". It means that the target is making himself seem foolish or that he is embarrassing himself.
"Josh is always making an ass out of himself when he's had too much to drink"
"Hey Josh! Stop making an ass out of yourself!"
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