When you shit on someone's dick and they anally fuck you, same goes for getting your dick shit on and anally fucking them. Also could be used as an insult.
Person 1: Yo, I just got shitdick'd today.
Person 2: what the fuck? You are actually disgustingwhat the hell, why do you want shit on your cock dude?
A chocolate cruller. A cruller is a doughnut style pastry that's twisted around itself and about eight inches long. It's irregular in shape and kind of lumpy, like a long bit of human feces. Yet it is long, like an erect penis. Hense the name "Shitdick."