Do you have a rival? Perhaps someone that you keep an eye on because you can’t trust what they say? This adjective, from the Latin “perfidia” (meaning “treachery”), would aptly describe your work nemesis or the former friend who betrayed your trust. The Latin root can be broken into its parts: “per-” (“detrimental to”) and “fides” (faith). However this “faith” is not a belief in a certain religion; instead it means “fidelity (or adherence) to one’s promises. A perfidious person is likely to break their promises. The noun form, “perfidy,” meaning “the quality or state of being faithless or disloyal.”