What does perfidious mean?
Perfidious describes a person or thing that violates trust, usually through deception or betrayal. The word is strongly negative: it suggests not merely disagreement or open opposition, but faithlessness concealed behind an appearance of loyalty. A perfidious ally may promise support while acting against the person who has trusted them; a perfidious action may disguise harmful intent as concern or fidelity. Because the accusation carries an implication of treachery, it is usually reserved for serious and deliberate conduct rather than ordinary mistakes or minor disappointments. The adjective is most often found in formal, literary, or historical prose, but it remains clear and useful in modern description. Related forms include the noun perfidy, meaning treachery or betrayal, and the adverb perfidiously, describing conduct that is faithless or deceitful. Its Latin ancestry combines “through” with “faithful,” reflecting the idea of fidelity turned against the person who trusted it.
adjectiveFaithless, treacherous, and deceitful, especially in a relationship where trust has been betrayed.
- Disloyal and treacherous, especially as a person who betrays trust through deception.
"The perfidious ally promised to support the prince while secretly informing his enemies."
"After years of apparent loyalty, his perfidious behavior became clear."
"The novel portrays a perfidious court official who uses lies to advance her own position."
Built from Latin roots meaning “through” and “faithful,” perfidious captures trust turned against the person who placed it.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does perfidious come from?
Perfidious comes through French from Latin perfidus, meaning “treacherous” or “faithless.” Latin perfidus was formed from per-, meaning “through,” and fidus, meaning “faithful,” with the resulting sense implying faithlessness or treachery. Its related forms include the English noun perfidy and cognates such as confide and confidence, all connected with the Latin root for trust or faithfulness.
Related word forms
How perfidious is actually used
Strongly disapproving and usually formal or literary. It suggests deliberate betrayal or deceit rather than ordinary disagreement, error, or broken expectations.