WordNet (r) 3.0 (2006):

goon
    n 1: an awkward stupid person [syn: lout, clod,
         stumblebum, goon, oaf, lubber, lummox, lump,
         gawk]
    2: an aggressive and violent young criminal [syn: hood,
       hoodlum, goon, punk, thug, tough, toughie,
       strong-armer]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "goon":
   Mafioso, Young Turk, adherent, battler, beast, beldam, belligerent,
   belted knight, berserk, berserker, bickerer, blade, bomber, bravo,
   brawler, bruiser, brute, bully, bullyboy, combatant, competitor,
   contender, contestant, creature, demon, devil, disciple, disputant,
   dragon, duelist, dummy, enforcer, fencer, feuder, fiend, fighter,
   fighting cock, figurehead, fire-eater, firebrand, flunky,
   flying squadron, foilsman, follower, fury, gamecock, gillie,
   gladiator, goon squad, gorilla, gun, gunsel, hanger-on, hardnose,
   hatchet man, hell-raiser, hellcat, hellhound, hellion, henchman,
   holy terror, hood, hoodlum, hooligan, hothead, hotspur, incendiary,
   jackal, jouster, killer, knight, lackey, mad dog, madcap, man,
   militant, minion, monster, mug, mugger, muscle man, myrmidon,
   picket, plug-ugly, puppet, quarreler, rapist, revolutionary,
   rioter, rival, rodman, rough, roughneck, rowdy, ruffian, sabreur,
   satellite, savage, scrapper, scuffler, she-wolf, spitfire,
   squabbler, stooge, strong arm, strong-arm man, strong-armer,
   struggler, swashbuckler, sword, swordplayer, swordsman, sycophant,
   termagant, terror, terrorist, thug, tiger, tigress, tilter,
   torpedo, tough, tough guy, trigger man, tussler, ugly customer,
   violent, virago, vixen, votary, wild beast, witch, wolf,
   wrangler