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):

nefariously
    adv 1: in a nefarious manner or to a nefarious degree;
           "nefariously involved in a conspiracy"