When you start fisting a girl in her pussy and then, with your other free hand, start fisting her up the asshole until your two hands meet inside her resulting in a "fist bump"
Yo bro i was fist bumping my girl last week, I felt my knucklestouch inside her
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