Per-fĭd'i-oŭs, a.
[Lat. perfidiosus, It. & O. Sp. perfidioso.]
Sense 1. Guilty of, or involving, perfidy; violating good faith or vows; false to trust or confidence reposed; treacherous; faithless; false; dishonest; as, a perfidious agent; a perfidious friend. “A most perfidious slave.” Shak.
Sense 2. Proceeding from treachery, or consisting in breach of faith; as, a perfidious act. “Involved in this perfidious fraud.” Milton.
Sense 3. Guilty of violated allegiance; as, a perfidious citizen; a man perfidious to his country.
Synonyms—Treacherous; faithless; unfaithful; false-hearted; disloyal; traitorous.