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1.
A person with a face like a mum or dinner lady; typically plain, old or generally unprepossessing.

Mum Face (n) or Mum Faced (adj) // Mom Faced (U.S)
Boy #1: That girl has a really hot body.

Boy #2: Yeah, but she's totally mum faced.
by PeaHead October 22, 2013
 
2.
Someone/thing who perpetrates/is (an act of) extreme annoyance.
Matthew Osman - Case in point.
Me: Hey, you spilled egg whites on the floor!

Osman: Yeah, and what are you going to do about it?

Me: you mumface! incidentally, i dislike you heartily. relocate yourself to a new locus.
by Jizzle on My Shanizzles January 23, 2007