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"

Moby Thesaurus II by Grady Ward, 1.0:

290 Moby Thesaurus words for "mug":
   Photostat, Telephoto, Wirephoto, X-ray, Xerox, aerial photograph,
   ambush, assail, assault, attack, babe, barbarize, batter, bazoo,
   beaker, black-and-white photograph, blackjack, blitz, blockhead,
   boob, bowl, bruiser, brutalize, bullyboy, burn, bushwhack, butcher,
   butt, byword, byword of reproach, calotype, candid photograph,
   capture on film, carry on, chalice, chaps, cheesecake, chops,
   chronophotograph, chump, cinch, coffee cup, color photograph,
   color print, come at, come down on, commit robbery, countenance,
   crack down on, credulous person, cull, declaim, derision,
   descend on, descend upon, destroy, dial, diapositive, dimwit, dolt,
   dope, dumbbell, dupe, easy mark, easy pickings, eggcup, embouchure,
   face, facies, fair game, fall guy, fall on, fall upon, favor,
   features, figure of fun, film, fish, fool, gab, game, gang up on,
   gazingstock, glass, go at, go for, go on, goat, gob, gobe-mouches,
   goblet, goon, gorilla, greener, greenhorn, greeny, grimace,
   gudgeon, gull, gun, gunsel, ham, ham it up, hammer, harry,
   hatchet man, have at, heist, heliochrome, heliograph, hellion,
   highball glass, hijack, hit, hit like lightning, hold up,
   holy terror, hood, hoodlum, hooligan, horn, idiot, ignoramus,
   innocent, jaw, jaws, jest, jestingstock, jigger, joke, jowls, jump,
   kisser, knock over, land on, laughingstock, lay at, lay hands on,
   lay into, lay waste, leadpipe cinch, light into, lineaments, lips,
   loot, loving cup, make a face, make a mouth, mandibles, map, mark,
   maul, maw, maxilla, microfilm, mockery, monkey, montage, mop,
   mop and mow, moue, mouth, mouthing, mow, mucker, mug shot, mugger,
   muscle man, mush, muzzle, noggin, numskull, oral cavity,
   out-herod Herod, overact, overdramatize, pan, patsy, phiz, photo,
   photobiography, photochronograph, photograph, photomap,
   photomicrograph, photomontage, photomural, physiognomy, picture,
   pigeon, pillage, pinup, pitch into, plaything, plug-ugly, pony,
   portrait, pounce upon, pound, pout, premaxilla, prize sap,
   pull a face, punk, pushover, puss, radiograph, rage, ramp, rampage,
   rant, rape, rave, riot, rip off, roar, rob, rodman, roentgenograph,
   rough, roughneck, rowdy, ruffian, ruin, sack, sail into, sap,
   saphead, savage, schlemiel, schooner, schooper, set on, set upon,
   shell, shoot, shot, shot glass, sitting duck, slaughter, slide,
   snap, snapshoot, snapshot, sow chaos, spout, stein, stick up,
   still, still photograph, stock, stooge, storm, strike,
   strong-arm man, sucker, surprise, swoop down on, take a photograph,
   take the offensive, talbotype, tankard, target, tassie, teacup,
   tear, tear around, telephotograph, terror, terrorize, throw away,
   thug, torpedo, tough, toy, transparency, trap, trigger man,
   trusting soul, tumbler, ugly customer, underact, vandalize, victim,
   violate, visage, wade into, wineglass, wreck, yap