Merriam-Webster Collegiate Dictionary - faithless

 adjective  Date: 14th century  1. not true to allegiance or duty ; treacherous, disloyal a ~ servant  2. not to be relied on ; untrustworthy a ~ tool  • ~ly adverb  • ~ness noun Synonyms:  ~, false, disloyal, traitorous, treacherous, perfidious mean untrue to what should command one's fidelity or allegiance. ~ applies to any failure to keep a promise or pledge or any breach of allegiance or loyalty ~ 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 ~ the implication of an incapacity for fidelity or reliability a perfidious double-crosser.