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.
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         The ambition and perfidy of tyrants.     --Macaulay.
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         His perfidy to this sacred engagement.   --DeQuincey.
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