perfidious
/(ˌ)pər-ˈfi-dē-əs/
adjective
of, relating to, or characterized by perfidy
faithless false disloyal traitorous treacherous perfidious mean untrue to what should command one's fidelity or allegiance. faithless applies to any failure to keep a promise or pledge or any breach of allegiance or loyalty. false stresses the fact of failing to be true in any manner ranging from fickleness to cold treachery.