Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

nefariousness
     n : the quality of being wicked [syn: {wickedness}, {vileness}]