Definition: 1: the quality or state of being faithless or disloyal : treachery  2: an act or an instance of disloyalty

Etymology: Latin perfidia, from perfidus faithless, from per- detrimental to + fides faith

Quotation:

When the facts started surfacing in the ’60s, his sole comment was two dry lines about the duke in his diary: “Secret papers have disclosed his pro-Nazi perfidy, which, of course, I was perfectly aware of at the time. … What a monumental ass he has always been!”

- from “The Playboy was a Spy”, an essay in The New York Times