The Collaborative International Dictionary of English v.0.48:

Barbarous \Bar"ba*rous\, a. [L. barbarus, Gr. ba`rbaros,
   strange, foreign; later, slavish, rude, ignorant; akin to L.
   balbus stammering, Skr. barbara stammering, outlandish. Cf.
   Brave, a.]
   1. Being in the state of a barbarian; uncivilized; rude;
      peopled with barbarians; as, a barbarous people; a
      barbarous country.
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   2. Foreign; adapted to a barbaric taste. [Obs.]
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            Barbarous gold.                       --Dryden.
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   3. Cruel; ferocious; inhuman; merciless.
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            By their barbarous usage he died within a few days,
            to the grief of all that knew him.    --Clarendon.
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   4. Contrary to the pure idioms of a language.
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            A barbarous expression                --G. Campbell.
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   Syn: Uncivilized; unlettered; uncultivated; untutored;
        ignorant; merciless; brutal. See Ferocious.
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WordNet (r) 3.0 (2006):

barbarous
    adj 1: (of persons or their actions) able or disposed to inflict
           pain or suffering; "a barbarous crime"; "brutal
           beatings"; "cruel tortures"; "Stalin's roughshod
           treatment of the kulaks"; "a savage slap"; "vicious
           kicks" [syn: barbarous, brutal, cruel, fell,
           roughshod, savage, vicious]
    2: primitive in customs and culture

Moby Thesaurus II by Grady Ward, 1.0:

174 Moby Thesaurus words for "barbarous":
   Doric, Draconian, Gothic, Neanderthal, Philistine, Tartarean,
   alien, animal, anthropophagous, atrocious, backward, barbarian,
   barbaric, beastly, benighted, bestial, bloodthirsty, bloody,
   bloody-minded, bookless, brutal, brutalized, brute, brutish,
   cacophonous, cannibalistic, careless, clumsy, coarse, cretinous,
   crude, cruel, cruel-hearted, deceived, demoniac, demoniacal,
   devilish, diabolic, doggerel, dysphemistic, erroneous, exotic,
   exterior, external, extraneous, extraterrestrial, extrinsic,
   faulty, fell, feral, ferine, ferocious, fiendish, fiendlike,
   fierce, foreign, foreign-born, functionally illiterate, graceless,
   grammarless, gross, harsh, heathen, hellish, hoodwinked, ignorant,
   ill-bred, ill-educated, illiterate, impolite, imprecise, improper,
   impure, in bad taste, inconcinnate, inconcinnous, incorrect,
   indecorous, inelegant, infelicitous, infernal, inhuman, inhumane,
   intrusive, kill-crazy, led astray, loose, low, lowbrow, malign,
   malignant, merciless, misinformed, misinstructed, mistaught,
   murderous, noncivilized, nonintellectual, outland, outlandish,
   outside, pagan, philistine, pitiless, primitive, rough-and-ready,
   rude, ruthless, sadistic, sanguinary, sanguineous, satanic, savage,
   sharkish, slavering, slipshod, slovenly, solecistic, strange,
   subhuman, tameless, tasteless, troglodytic, truculent, ulterior,
   unbooked, unbookish, unbooklearned, unbriefed, unchristian,
   uncivil, uncivilized, uncombed, unconscionable, uncourtly, uncouth,
   uncultivated, uncultured, undignified, unearthly, unedified,
   uneducated, unerudite, uneuphonious, unfelicitous, ungentle,
   ungodly, ungraceful, ungrammatic, unguided, unholy, unhuman,
   uninstructed, unintellectual, unkempt, unlearned, unlettered,
   unlicked, unliterary, unpolished, unread, unrefined, unscholarly,
   unschooled, unseemly, unstudious, untamed, untaught, untutored,
   vicious, vulgar, wicked, wild, wolfish