Of, pertaining to, or representing perfidy; disloyal to what should command one's fidelity or allegiance.

(rare) Unfaithfulness; deceitfulness; perfidy.

In a perfidious manner.

plural of perfidiousness

A state or act of violating faith or allegiance; violation of a promise or vow, or of trust

plural of perfidy

(poetic, derogatory) The United Kingdom, seen as a country that is manipulative, deceptive, or treacherous on the international stage.

Alternative letter-case form of Perfidious Albion.