When a meat-head or bro tackles a smaller person so that he can violently rub his head on their head. The purpose of the flat dog may be homoerotic in nature.
Sean, stop flat dogging Kyle. He's about to cry and you have a Texas-sized erection.
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