2 girls 1 cup owes it's origins to a film sharing the same title in which two young girls of appropriate age engage in socialy questionable acts including: consumption of one another's feces, consumtion of the regurgitated aforementioned feces, and aside from the shit and puke eating some pretty hot lesbian action. This having been explained, its defined currently as a description of anything traumatic and disgusting enough to shock, often relating to but not exclusive to the consumtion of vomit or feces.
Reginald: I say I say good sir have you seen the cinema film "The Human Centipede"? Farnsworth: No my good man I've not seen this feature would you recomend I take my best girl and her father? Reginald: Well my gentle sir I'd say that shits 2 girls 1 cup!