While having diarrhea, you take a shit on someone's chest (preferably a female so the breasts are utilized as a canal) and then the person stands up. Allowing the runny defecate to act as a mudslide, running down the torso.
I was on a cheap domestic beer binge and your mom let me demonstrate a Bavarian Mudslide on her for the party entertainment.
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi