When one takes a shit and doesn't clean themselves, it dries, and another then has anal sex with the dry, dirty, poop butt.
"After drinking and chinese food last night, we got back to my place and (i didn't have and toilet paper) we started getting freaky, needless to say i got the crusty muffler."
A crusty muffin is a slang term for a very unclean vagina. Typically with flakes of crusty skin, dried semen, dried discharge, or any other patina of std origin.