perfidious

This one is basically faith with a knife in its back. Latin speakers had fides for trust and per for “through” or “completely,” and when those pieces got welded into perfidia, the result was not mere unreliability but full-blown faithlessness. By the late 1590s English had turned Latin perfidiosus into perfidious, and the word arrived already wearing a cape and a sneer. That same trust-family gives us fidelity, confide, and fiduciary, so perfidious feels like the evil twin at the family reunion. Napoleon even helped make “perfidious Albion” famous in 1813, a phrase that paints Britain as the ultimate backstabber. Say it once and you can almost hear the door closing behind the betrayal.