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1.
When someone with a mustache (male or female), administers oral sex on a female and lets the vaginal fluids crustify onto the mustache.
"Jimbo grew a mustache in anticipation for his inaugural crusty muffin."
by The PGP January 18, 2010
 
2.
To eat a girl out. As in oral sexual intercourse with a female. Commonly related to a "muff dive".
Matt:"Dude, I totally ate a crusty muffin last night!"
Kevin:"Really, was it warm, fresh and tasty? Did you spell out the alphabet??"
by Matt (Crusty Muffin Man) May 16, 2007