I came across a list of homonyms: mu, moo, moue. I know mu is Greek for the letter m, and moo is the sound cows make, but what's a moue?

Moue is a French word for a slightly pouting face.

Check out this passage from Oscar Wilde's classic novel, The Picture of Dorian Gray:

Dorian Gray stepped up on the dais with the air of a young Greek martyr, and made a little moue of discontent to Lord Henry, to whom he had rather taken a fancy.