Mug is a funny word that could represent a cup, your face, or even what you do when you rob someone. And, if you mug someone for their mug, you may end up getting a mug shot taken of your guilty face. Confusing? Read on.
A mug is an informal vessel for drinking coffee or tea, as opposed to a china tea cup or a demitasse espresso cup, for example. Informally, a mug is a face: that's why a mug shot of a prisoner is a picture of his face right after he has been arrested. When it's a verb, mug means "rob someone while threatening violence." This meaning came from an earlier definition, "to strike someone in the face (or the mug)."