adjective per·fid·i·ous \ (ˌ)pər-ˈfi-dē-əs \
|Updated on: 21 May 2018

Definition of perfidious

: of, relating to, or characterized by perfidy





perfidious was our Word of the Day on 07/11/2010. Hear the podcast!

Examples of perfidious in a Sentence

  1. We were betrayed by a perfidious ally.

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

Recent Examples of perfidious from the Web

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'perfidious.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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.

First Known Use of perfidious


in the meaning defined above

See Words from the same year
NEW! Time Traveler

Synonym Discussion of 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

PERFIDIOUS Defined for English Language Learners


Definition of perfidious for English Language Learners

  • : not able to be trusted : showing that someone cannot be trusted

Seen and Heard

What made you want to look up perfidious? Please tell us where you read or heard it (including the quote, if possible).

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!


to grant as a privilege or special favor

Get Word of the Day daily email!

Test Your Vocabulary

Obscure Shapes

  • a-pile-of-three-dimensional-shapes-in-green
  • Something that is ooid is shaped like:
Name That Thing

Test your visual vocabulary with our 10-question challenge!


Test Your Knowledge - and learn some interesting things along the way.


Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!