perfidious

American  

[per-fid-ee-uhs] / pərˈfɪd i əs /

adjective

1. deliberately faithless; treacherous; deceitful.

   a perfidious lover.

   Synonyms:

   traitorous, unfaithful, disloyal, false

   Antonyms:

   faithful

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perfidious British  

/ pəˈfɪdɪəs /

adjective

1. guilty, treacherous, or faithless; deceitful

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Other Word Forms

• perfidiously adverb
• perfidiousness noun
• unperfidious adjective
• unperfidiously adverb
• unperfidiousness noun

Etymology

Origin of perfidious

First recorded in 1530–40; from Latin perfidiōsus “faithless, dishonest”; perfidy, -ous

Example Sentences

Samuel Pepys knew him; the British diarist thought him “a perfidious rogue.”

From The Wall Street Journal • Mar. 11, 2026

The head of one of them, who preferred to remain anonymous, described it as a "perfidious and effective" strategy.

From Barron's • Nov. 1, 2025

But in the Disney+ series “Agatha All Along,” Hahn pulls from all the disparate strands of her body of work to play the perfidious, power-hungry witch Agatha Harkness.

From Los Angeles Times • Nov. 26, 2024

“If the initial findings are confirmed, it will be a perfidious attack on our electricity infrastructure,” Stübgen said.

From Seattle Times • Mar. 5, 2024

There was an element of chauvinism in this—the French supported their own champion and rejected the ideas of the perfidious Englishman, while Newton was, of course, very much a prophet honoured in his own country.

From "The Scientists" by John Gribbin