The Collaborative International Dictionary of English v.0.48:

Nefarious \Ne*fa"ri*ous\, a. [L. nefarius, fr. nefas crime,
   wrong; ne not + fas divine law; akin to fari to speak. See
   No, adv., and Fate.]
   Wicked in the extreme; abominable; iniquitous; atrociously
   villainous; execrable; detestably vile.
   [1913 Webster]

   Syn: Iniquitous; detestable; horrible; heinious; atrocious;
        infamous; impious. See Iniquitous.
        [1913 Webster] -- Ne*fa"ri*ous*ly, adv. --
        Ne*fa"ri*ous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

nefarious
    adj 1: extremely wicked; "nefarious schemes"; "a villainous
           plot"; "a villainous band of thieves" [syn: nefarious,
           villainous]

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "nefarious":
   abject, abominable, arrant, atrocious, awful, bad, base, beastly,
   beggarly, beneath contempt, black, blamable, blameworthy, brutal,
   cheesy, contemptible, corrupt, criminal, crummy, damnable, dark,
   debased, degenerate, degraded, deplorable, depraved, despicable,
   detestable, dire, dirty, disgraceful, disgusting, dreadful,
   egregious, enormous, evil, execrable, fetid, filthy, flagitious,
   flagrant, foul, fulsome, glaring, grave, grievous, gross, hateful,
   heinous, horrible, horrid, improper, infamous, iniquitous, knavish,
   lamentable, little, loathsome, lousy, low, low-down, lumpen, mangy,
   mean, measly, miscreant, miserable, monstrous, nasty, naughty,
   noisome, notorious, obnoxious, odious, offensive, outrageous,
   paltry, peccant, perverse, petty, pitiable, pitiful, poky, poor,
   putrid, rank, regrettable, reprehensible, reprobate, reptilian,
   repulsive, rotten, sad, scabby, scandalous, schlock, scrubby,
   scruffy, scummy, scurvy, shabby, shameful, shocking, shoddy,
   sinful, small, sordid, squalid, terrible, too bad, unclean,
   unforgivable, unmentionable, unpardonable, unspeakable, unworthy,
   vicious, vile, villainous, wicked, woeful, worst, worthless,
   wretched, wrong