When a person goes from jovial and friendly to furious and belligerent at the flip of a switch.
Mitchell and Robert were about to shagLena when her sister, Summer, all of the sudden went apeshit then Yahtzee to Nazi on them, thus having shit on Mitchell and Robert's chances to hook up with Lena.
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