From The Collaborative International Dictionary of English v.0.48:
Mug \Mug\ (m[u^]g), n. [Cf. Ir. mugam a mug, mucog a cup.]
1. A kind of ceramic or metal drinking cup, with a handle, --
usually cylindrical and without a lip.
[1913 Webster]
2. The face or mouth; as, I don't want to see your ugly mug
again; -- often used contemptuously. [Slang] --Thackeray.
[1913 Webster +PJC]
From The Collaborative International Dictionary of English v.0.48:
mug \mug\ (m[u^]g), v. t.
To take property from (a person) in a public place by
threatening or committing violence on the person who is
robbed; to rob, especially to rob by use of a weapon such as
a knife or gun.
Note: To rob a person or a business indoors is not usually
referred to as to mug, but to stick up or hold up.
Syn: rob, stick up
[WordNet 1.5 +PJC]