faithless

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>.

faithless, disloyal, and traitorous mean not being true to something that has a right to a person's loyalty. faithless is used when a person breaks a promise or pledge to remain loyal to someone or something. <Our faithless friends left us at the first sign of trouble.> disloyal is used when a person is unfaithful to someone or something that has the right to expect loyalty. <The disloyal citizens will be punished.> traitorous is used for actual treason or a betrayal of trust. <The traitorous soldier was giving secrets to the enemy.>