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

Explanation

If someone accuses you of being perfidious, you should probably be offended — it means underhanded, treacherous, deceitful — even evil. If you betray people often, you're perfidious: traitors are extremely perfidious. Besides betrayal, this word implies lying and maybe other kinds of awful behavior, like stealing and taking bribes. Everyone tries to avoid perfidious people. Perfidious is kind of an old-fashioned word, but being perfidious will never go out of style, unfortunately.

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

“Behold, campers, this vile and destructive creature! This killer of songbirds! This murderer of chipmunks! This perfidious predator!”

From "Klawde: Evil Alien Warlord Cat" by Johnny Marciano and Emily Chenoweth