per·​fid·​i·​ous (ˌ)pər-ˈfi-dē-əs How to pronounce perfidious (audio)
: 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.

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 In what might sound to many Americans like a familiar tactic, Netanyahu cast his critics and not himself as a threat to the country’s security, and sought to position himself as a victim of perfidious judges and prosecutors. Tracy Wilkinson, Los Angeles Times, 27 Mar. 2023 But writer Rob Engalla’s perfidious hand movements were their undoing. Vulture, 10 Mar. 2023 So much of what citizens know of our perfidious former president has come from her writing and reporting. Keith L. Runyon, The Courier-Journal, 17 Nov. 2022 But showing care for others, in the Republican party’s political trauma era, is a perfidious weakness. Rich Logis, The New Republic, 13 Oct. 2022 Marsha’s career was derailed by the Blacklist, a perfidious period of American history that has been endlessly chronicled and misunderstood. Alan K. Rode, Variety, 10 Sep. 2022 On the world stage, Ukraine consistently portrays Russia as a perfidious power that cannot be trusted to honor international agreements — and Moscow’s actions often make that characterization compelling. Laura King, Los Angeles Times, 24 July 2022 This latest version, originating in the Kremlin, is just a new, perfidious variant of the old virus called anti-Semitism. Michael Brenner, Smithsonian Magazine, 29 June 2022 But what’s truly unprecedented about this bill is the range of organizations that its supporters hope to cleanse of perfidious foreign influence. Casey Michel, The New Republic, 27 June 2022 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'perfidious.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

First Known Use

1572, in the meaning defined above

Time Traveler
The first known use of perfidious was in 1572


Dictionary Entries Near perfidious

Cite this Entry

“Perfidious.” Dictionary, Merriam-Webster, Accessed 8 Dec. 2023.

Kids Definition


per·​fid·​i·​ous (ˌ)pər-ˈfid-ē-əs How to pronounce perfidious (audio)
: of, relating to, or characterized by perfidy : treacherous
perfidiously adverb
perfidiousness noun

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