Faithless, false, disloyal, traitorous, treacherous, perfidious mean untrue to a person, an institution, or a cause that has a right to expect one’s fidelity or allegiance.
Faithless applies to a person, utterance, or act that implies a breach of a vow, a pledge, a sworn obligation, or allegiance. Although often used interchangeably with the strongest of the terms here discriminated, then implying a betrayal of a person or cause, it is also capable of implying untrustworthiness, unreliability, or loss or neglect of an opportunity to prove one’s devotion or faith.
False differs from faithless in its greater emphasis upon a failure to be true or constant in one’s devotion or adherence than upon an actual breach of a vow, pledge, sworn promise, or obligation; however it may, like faithless, carry varying connotations with respect to the gravity or heinousness of that failure.
Disloyal implies lack of faithfulness in thought, in words, or in actions to one (as a friend, superior, sovereign, party, or country) to whom loyalty is owed.
Traitorous implies either actual treason or a serious betrayal of trust or confidence.
Treacherous is of wider application than traitorous; as used of persons it implies readiness, or a disposition, to betray trust or confidence <a treacherous ally> and as used of things it suggests aptness to lead on to peril or disaster by false or delusive appearances.
Perfidious is a more contemptuous term than treacherous; it implies baseness or vileness as well as an incapacity for faithfulness in the person concerned.