The Collaborative International Dictionary of English v.0.48:

Mugger \Mug"ger\, n. Also Muggar \Mug"gar\, Muggur \Mug"gur\
   [Hind. magar, fr. Skr. makara sea monster.]
   The common crocodile (Crocodilus palustris) of India, the
   East Indies, etc. It becomes twelve feet or more long.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

mugger \mugger\ n.
   A thief who takes property by threatening (or performing)
   violence on the person who is robbed; a person who commits a
   mugging; one who mugs. See mug, v. t.

   Syn: robber
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

mugger
    n 1: a robber who takes property by threatening or performing
         violence on the person who is robbed (usually on the
         street)

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "mugger":
   Mafioso, Young Turk, aggressor, assailant, assailer, assaulter,
   attacker, bank robber, beast, beldam, berserk, berserker, bomber,
   bookworm, bruiser, brute, bushranger, demon, devil, dragon,
   failing student, fiend, fire-eater, firebrand, footpad, fury, goon,
   gorilla, greasy grind, grind, gun, gunsel, hardnose, hatchet man,
   hell-raiser, hellcat, hellhound, hellion, highway robber,
   highwayman, hijacker, holdup man, holy terror, hood, hoodlum,
   hooligan, hothead, hotspur, incendiary, invader, killer, mad dog,
   madcap, monster, mug, muscle man, overachiever, plug-ugly, raider,
   rapist, revolutionary, road agent, robber, rodman, roughneck,
   sandbagger, savage, she-wolf, spitfire, stickup man,
   strong-arm man, swotter, termagant, terror, terrorist, tiger,
   tigress, torpedo, tough, tough guy, train robber, trigger man,
   ugly customer, underachiever, violent, virago, vixen, wild beast,
   witch, wolf