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.
I almost picked up a hooker last night. I saw she had a crusty muffin, so I had to flee.
When the anal cavity is penetrated with such viciousness that it pulls the rectal out; creating a hemroid that resembles the top of a muffin that is bloody and crusty with semen.