From Stephen King’s (et al.) The Dark Tower: Treachery
Perfidy: An act of deceit; violating faith, trust, promises, vows etc. It is very fitting when a young Roland Deschain uses this word to describe his mother. Perfidy can be equated to treason or treachery.
It’s interesting that they’ve used a synonym for treachery in the novel, a word very fitting with the atmosphere of the story. Would I ever use this word? First I’d have to learn how to pronounce it without it sounded a little silly.