When something is so unbelievable that the phrase "are you shitting me?" is not enough. It demonstrates that the action taking place, or the event being described is so monstrously horrific that it literally feels like someone is taking a massive shit inside of your chest. Basically, the worst feeling ever is happening, and at this point you're questioning why the fuck.
Douche: LOL I TOTALLY RAPED THIS CHILD WITH A PITCHFORK OF AIDS Person: ...are you shitting inside of me?
Douche: ...No? But I could be.
Ruler of Lake Titicaca. Rumored to have a bunghole that gets very angry if it does not receive toilet paper. Cornholio the Great is often seen walking around with his shirt over his head and his hands in the air, chanting songs about his power, and his bunghole.
"I am Cornholio! You do not want to face the wrath of my bunghole, for I need TP!"
Butthead: Shut up, Beavis! (uh huh huh huh)
Beavis: Um, okay. (heh heh heh heh).