a. [ L. perfidious. ] 1. Guilty of perfidy; violating good faith or vows; false to trust or confidence reposed; treacherous; faithless; as, a perfidious friend. Shak. [ 1913 Webster ]

2. Involving, or characterized by, perfidy. “Involved in this perfidious fraud.” Milton. [ 1913 Webster ]

adv. In a perfidious manner. [ 1913 Webster ]

n. The quality of being perfidious; perfidy. Clarendon. [ 1913 Webster ]