The Collaborative International Dictionary of English v.0.48:

Perfidy \Per"fi*dy\ (p[~e]r"f[i^]*d[y^]), n.; pl. Perfidies
   (p[~e]r"f[i^]*d[i^]z). [L. perfidia, fr. L. perfidus
   faithless; per (cf. Skr. par[=a] away) + fides faith: cf. F.
   perfidie. See Faith.]
   The act of violating faith or allegiance; violation of a
   promise or vow, or of trust reposed; faithlessness;
   treachery.
   [1913 Webster]

         The ambition and perfidy of tyrants.     --Macaulay.
   [1913 Webster]

         His perfidy to this sacred engagement.   --DeQuincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

perfidy
    n 1: betrayal of a trust [syn: perfidy, perfidiousness,
         treachery]
    2: an act of deliberate betrayal [syn: treachery, betrayal,
       treason, perfidy]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "perfidy":
   betrayal, deceit, dirty pool, dirty trick, dirty work, disloyalty,
   double-dealing, duplicity, faithlessness, falseheartedness,
   falseness, falsity, foul play, hypocrisy, infidelity,
   insidiousness, perfidiousness, sellout, traitorousness,
   treacherousness, treachery, treason, unfaithfulness