Emily Dickinson Lexicon

PER-FID'I-OUS, a. [L. perfidus; per and fidus, faithful. Per in this word signifies through, beyond, or by, aside.]

1. Violating good faith or vows; false to trust or confidence reposed; treacherous; as, a perfidious agent; a perfidious friend. [See Perfidy.]
2. Proceeding from treachery, or consisting in breach of faith; as, a perfidious act.
3. Guilty of violated allegiance; as, a perfidious citizen; a man perfidious to his country.