Shit On My Ass (etymology: Mod. English; portmaneau of the phrases "shit" and "ass," with "on my" forming the liaison) is a vulgar phrase expressing an utter discontent in an occurence, (e.g. where the conjuction "Shit!" would be used). Offshoots may include: shit on my dick, shite on my arse, holy shit on my ass, and the extremely long "piece of shit motherfucker on my ass."
Teacher: You failed the test, Stan.
Stan: Shit on my ass!
Brittany: I'm cheating on you, Stan.
Stan: You piece of shit motherfucker on my ass!