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. faithless allies false stresses the fact of failing to be true in any manner ranging from fickleness to cold treachery. betrayed by false friends disloyal implies a lack of complete faithfulness to a friend, cause, leader, or country. disloyal to their country traitorous implies either actual treason or a serious betrayal of trust. traitorous acts punishable by death treacherous implies readiness to betray trust or confidence. a treacherous adviser perfidious adds to faithless the implication of an incapacity for fidelity or reliability. a perfidious double-crosser

Did You Know?

We wouldn't lie to you about the history of "perfidious" - even though the word itself suggests deceitfulness. The modern English meaning of "perfidious" remains faithful to that of its Latin ancestor, perfidus, which means "faithless." English speakers have used "perfidious" to mean "treacherous" since at least 1572. One of the earliest known uses of the term can be found in Act V, scene iii of Shakespeare’s All's Well That Ends Well: the "perfidious slave" Parolles is thought to be an unreliable witness; he’ll say whatever suits his purpose, whether true or not. In contemporary usage, "perfidious" not only implies treacherousness, but an inability to be reliable or honorable.