The Collaborative International Dictionary of English v.0.48:

Thug \Thug\ (th[u^]g; 277), n. [Hind. thag a deceiver, robber.]
   1. One of an association of robbers and murderers in India
      who practiced murder by stealthy approaches, and from
      religious motives. They have been nearly exterminated by
      the British government.
      [1913 Webster]

   2. An assassin; a ruffian; a rough. "Thugs and midnight
      rounders." --The Century.
      [Webster 1913 Suppl.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

thug
    n 1: an aggressive and violent young criminal [syn: hood,
         hoodlum, goon, punk, thug, tough, toughie,
         strong-armer]

Moby Thesaurus II by Grady Ward, 1.0:

154 Moby Thesaurus words for "thug":
   Cain, Judas, Mafioso, adherent, apache, assassin, assassinator,
   bandit, battler, belligerent, belted knight, betrayer, bickerer,
   blade, bloodletter, bloodshedder, bravo, brawler, brigand, bucko,
   bully, bullyboy, burker, butcher, button man, cannibal, combatant,
   competitor, contender, contestant, convict, creature, criminal,
   crook, cutthroat, dacoit, deceiver, desperado, desperate criminal,
   devil, disciple, disputant, double-dealer, duelist, dummy,
   enforcer, eradicator, executioner, exterminator, felon, fencer,
   feuder, fighter, fighting cock, figurehead, flunky, foilsman,
   follower, fugitive, gallows bird, gamecock, gangster, gaolbird,
   garroter, gillie, gladiator, goon, gorilla, gun, gunman, gunsel,
   hanger-on, hatchet man, head-hunter, heavy, hell-raiser, hellcat,
   henchman, hit man, homicidal maniac, homicide, hood, hoodlum,
   hooligan, jackal, jailbird, jouster, killer, knight, lackey,
   lawbreaker, mad dog, man, man-eater, man-killer, manslayer,
   massacrer, matador, militant, minion, mobsman, mobster, mugger,
   murderer, myrmidon, outlaw, pesticide, plug-ugly, poison, poisoner,
   public enemy, puppet, quarreler, quisling, racketeer, rioter,
   rival, robber, rough, rowdy, ruffian, sabreur, satellite, scofflaw,
   scrapper, scuffler, slaughterer, slayer, squabbler, stooge,
   strangler, strong, strong arm, strong-arm man, strong-armer,
   struggler, swashbuckler, swindler, sword, swordplayer, swordsman,
   sycophant, terrorist, thief, tilter, torpedo, tough, traitor,
   trigger man, tussler, two-timer, votary, wrangler, yahoo