A small grimace or pout. An expression of distaste or playful pouting.
This is the first time I ever looked up this word in a dictionary; I was rather surprised to find that every dictionary uses the word grimace in the definition somewhere. The words grimace and moue do not identify themselves in my mind at all. A moue is a small, probably cute and pouty, and often flirtatious pursing of the lips. A grimace is small ugly frown. Right? That seems to be what I see in common usage, anyway. Ah well. On with the writeup.
Moue is from the French for 'pout'1, and is pronounced 'mu' ('moo'). It comes from the same root (the Middle French 'moe') as mow, a 'more English' word meaning 'a mocking grimace or a derisive expression'. Moue probably was taken directly from the current French, in an attempt to appear sophisticated.
It is usually used with words like 'small', 'slight',
References:
1. This translation is from http://babelfish.altavista.com/tr, which translates moue into pout, but pout into tacaud.
http://dictionary.reference.com/search?q=moue&r=66
Webster's Third New International Dictionary