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]

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]

WordNet (r) 3.0 (2006):

mug
    n 1: the quantity that can be held in a mug [syn: mug,
         mugful]
    2: a person who is gullible and easy to take advantage of [syn:
       chump, fool, gull, mark, patsy, fall guy,
       sucker, soft touch, mug]
    3: the human face (`kisser' and `smiler' and `mug' are informal
       terms for `face' and `phiz' is British) [syn: countenance,
       physiognomy, phiz, visage, kisser, smiler, mug]
    4: with handle and usually cylindrical
    v 1: rob at gunpoint or with the threat of violence; "I was
         mugged in the streets of New York last night"