Source : Webster's Revised Unabridged Dictionary (1913)

Perfidious \Per*fid"i*ous\ (p[~e]r*f[i^]d"[i^]*[u^]s; 277), a.
   [L. perfidious.]
   1. Guilty of perfidy; violating good faith or vows; false to
      trust or confidence reposed; teacherous; faithless; as, a
      perfidious friend. --Shak.

   2. Involving, or characterized by, perfidy. ``Involved in
      this perfidious fraud.'' --Milton.

Source : WordNet®

perfidious
     adj : tending to betray; especially having a treacherous character
           as attributed to the Carthaginians by the Romans;
           "Punic faith"; "the perfidious Judas"; "the fiercest
           and most treacherous of foes"; "treacherous intrigues"
           [syn: {punic}, {treacherous}]