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

• traitorous
• false

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

Her Ashley is the movie’s fourth and fifth wheel, dismissed as both a perfidious troublemaker and a New Age airhead—a life coach in need of a wife coach. —Justin Chang, New Yorker, 22 Aug. 2025

This perfidious situation, unintentional as its creation may have been, was strongly critiqued as far back as 1983 by Jonathan Crush and Paul Wellings in an article in The Journal of Modern African Studies. —The Editors, JSTOR Daily, 15 Aug. 2025

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

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