When you shit on someone's dick and they anally fuck you, same goes for getting your dickshit 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 disgusting what 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."
"I stopped by the doughnut shop and bought us some shitdicks and coffee."