The muddy sheets is an exotic and erotic sexual maneuver, involving a man, woman, and the man's best friend. You begin by fucking your lady friend in the ass via doggy style while under the sheets of your best friend's bed. She begins to get tired out when you donkey punch her hopefully initiating the washing machine. IF the first punch does not grant the desired effect, punch repeatedly. Mid seizure she will begin shitting all over the covers, in which case you GET OUT OF THERE. Make sure to yell as loudly as possible when running away, kind of like a dying goat. This will create the muddy sheets effect for your best friend when he comes home and finds a passed out chick in a shit covered bed. Bon apetite!
Ethan was shocked and appauled to find Kasi, his ex-lover, in some muddy sheets, right after Kevin ran out of his room yelling "BAAAAGHGGHHHHHHAHAWH"