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.

false stresses the fact of failing to be true in any manner ranging from fickleness to cold treachery.

disloyal implies a lack of complete faithfulness to a friend, cause, leader, or country.

traitorous implies either actual treason or a serious betrayal of trust.

treacherous implies readiness to betray trust or confidence.

perfidious adds to faithless the implication of an incapacity for fidelity or reliability.