noun

perfidy

PER-fuh-dee/ˈpɜːrfədi/

Deliberate betrayal of trust or loyalty.

The caretaker's perfidy gives an outsider access entrusted by the homeowner.

A trusted caretaker secretly gives the household key to a waiting outsider while the homeowner turns away in confidence, clear deliberate betrayal of entrusted access.

Etymology

perLatinthrough

+fidLatinfaith, trust

+yLatinstate or quality

“Through trust” belongs to the fuller expression “to deceive through trustingness.” Perfidy therefore names betrayal that takes advantage of faith placed in someone, rather than trust itself.

In context

Libranima557:12

I scarcely restrained myself from rushing out into the streets, crying aloud and proclaiming openly the perfidy and treachery of which I was the victim

Don Quixote - Cervantes (Illustrated Audiobook) Part 1 of 4

More examples

He never forgives his closest friend's perfidy.
The ambassador calls the broken treaty an act of perfidy.
Behind her promises of loyalty lies a long history of perfidy.

Forms

perfidy · perfidies

perfidious adjective — Deliberately betraying someone's trust or loyalty.

Similar words

betrayal: Broader term for breaking trust, without the same formal tone.
treachery: Often suggests hidden plotting or betrayal that causes serious harm.
disloyalty: Can mean a lack of allegiance without a deliberate betrayal.