treacherous

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