perfidious

adjective

per·​fid·​i·​ous (ˌ)pər-ˈfi-dē-əs 

: of, relating to, or characterized by perfidy

perfidiously adverb

perfidiousness noun

Did you know?

We wouldn't lie to you about the history of "perfidious" - even though the word itself suggests deceitfulness. The modern English meaning of "perfidious" remains faithful to that of its Latin ancestor, perfidus, which means "faithless." English speakers have used "perfidious" to mean "treacherous" since at least 1572. One of the earliest known uses of the term can be found in Act V, scene iii of Shakespeare’s All's Well That Ends Well: the "perfidious slave" Parolles is thought to be an unreliable witness; he’ll say whatever suits his purpose, whether true or not. In contemporary usage, "perfidious" not only implies treacherousness, but an inability to be reliable or honorable.

Synonyms

• disloyal
• faithless
• false
• fickle
• inconstant
• recreant
• traitorous
• treacherous
• unfaithful
• untrue

Choose the Right Synonym for perfidious

faithless, false, disloyal, traitorous, treacherous, perfidious mean untrue to what should command one's fidelity or allegiance.

faithless applies to any failure to keep a promise or pledge or any breach of allegiance or loyalty.

faithless allies

false stresses the fact of failing to be true in any manner ranging from fickleness to cold treachery.

betrayed by false friends

disloyal implies a lack of complete faithfulness to a friend, cause, leader, or country.

disloyal to their country

traitorous implies either actual treason or a serious betrayal of trust.

traitorous acts punishable by death

treacherous implies readiness to betray trust or confidence.

a treacherous adviser

perfidious adds to faithless the implication of an incapacity for fidelity or reliability.

a perfidious double-crosser

Examples of perfidious in a Sentence

We were betrayed by a perfidious ally.

a perfidious campaign worker revealed the senator's strategy to his leading rival for the nomination

Recent Examples on the Web

Those voting for it are Sean Elo-Rivera, Jennifer Campbell, Kent Lee, Stephen Whitburn, Vivian Moreno and La Jolla’s own Joe LaCava, who has been an active supporter of this perfidious plan. —Inga, San Diego Union-Tribune, 16 June 2025

Jake Gyllenhaal is his foil as the perfidious Iago, who goads Othello into unreasoning jealousy with lies about his beloved Desdemona (Molly Osborne). —New York Times, 17 Feb. 2025

Since then, Putin has sought instead to rally the public to the defense of a motherland besieged by the perfidious and cunning West. —Leon Aron, The Atlantic, 5 Dec. 2024

The result is a cacophonous temper tantrum, a vacuous and perfidious advertisement for military recruitment. —Gregory Nussen, Deadline, 28 Mar. 2025

Word History

First Known Use

1572, in the meaning defined above

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Kids Definition

perfidious

adjective

per·​fid·​i·​ous (ˌ)pər-ˈfid-ē-əs 

: of, relating to, or characterized by perfidy : treacherous

perfidiously adverb

perfidiousness noun