1. The having of pre-pubescent facial hair above the lip.
2. A human of the female variety with a hairy upper lip.
3. The act of being a complete and utter hairy douchebag.
4. The act of gluing your partner's lips shut with your cum.
Ted:OMG! Todd,what the fuck is that on Mary's face?
Todd: I think it's a damn masteusche!
Grisly: "Grow some balls and shave that masteusche, nigger boy neal"
Captain America: "Wonder Boy, the next time I see your mom, I'm going to masteusche her lips together."
Masten is an wonderful, intriguing creature. He is always eating shrimp but be careful, if he eats too many he will fall into a deep hibernation for days (like a bear!).
ADDITIONAL WARNING: He can get a whole lot more retarted.