The Collaborative International Dictionary of English v.0.48:

Villainous \Vil"lain*ous\, a. [Written also villanous.]
   [1913 Webster]
   1. Base; vile; mean; depraved; as, a villainous person or
      wretch.
      [1913 Webster]

   2. Proceeding from, or showing, extreme depravity; suited to
      a villain; as, a villainous action.
      [1913 Webster]

   3. Sorry; mean; mischievous; -- in a familiar sense. "A
      villainous trick of thine eye." --Shak.
      [1913 Webster]

   Villainous judgment (O. E. Law), a judgment that casts
      reproach on the guilty person.
      [1913 Webster] --- Vil"lain*ous*ly, adv.
      Vil"lain*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

villainous
    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:

121 Moby Thesaurus words for "villainous":
   abandoned, abominable, arrant, atrocious, awful, bad, base,
   beastly, beneath contempt, bent, black, blackguardly, blamable,
   blameworthy, brutal, contemptible, contrary, corrupt, criminal,
   crooked, damnable, dark, debased, degenerate, degraded, deplorable,
   despicable, detestable, dire, disgraceful, disgusting, dishonest,
   dissolute, dreadful, egregious, enormous, evil, execrable,
   faithless, felonious, fetid, filthy, flagitious, flagrant, foul,
   fulsome, grievous, gross, hateful, heinous, horrible, horrid,
   improper, infamous, iniquitous, knavish, lamentable, loathsome,
   lousy, low, miscreant, monstrous, murderous, nasty, naughty,
   nefarious, noisome, notorious, objectionable, obnoxious, odious,
   offensive, outrageous, peccant, perfidious, perverse, perverted,
   pitiable, pitiful, profligate, putrid, rank, rascally, recreant,
   regrettable, reprehensible, reprobate, repulsive, roguish, rotten,
   sad, scampish, scandalous, schlock, scoundrelly, scurvy, shabby,
   shameful, shocking, shoddy, sinful, sordid, squalid, terrible,
   too bad, traitorous, treacherous, unclean, unforgivable,
   unpardonable, unscrupulous, unspeakable, unworthy, vicious, vile,
   wicked, woeful, worst, worthless, wretched, wrong