faithless implies failure to adhere, as to an oath or obligation a faithless wife; false, in this connection more or less synonymous with faithless, stresses failure in devotion to someone or something that has a moral claim to one's support a false friend; disloyal implies a breach of allegiance to a person, cause, institution, etc. disloyal to one's family; traitorous strictly implies the commission of treason; treacherous suggests an inclination or tendency to betray a trust his treacherous colleagues; perfidious adds to the meaning of treacherous a connotation of sordidness or depravity a perfidious informer
These adjectives mean not true to duty or obligation. Faithless and unfaithful imply failure to adhere to promises, obligations, or allegiances: was faithless to her ideals; an unfaithful spouse.False emphasizes deceitfulness: “To thine own self be true,/And it must follow, as the night the day,/Thou canst not then be false to any man” (Shakespeare). One who is disloyal betrays an allegiance: disloyal staff members who exposed the senator's indiscretions.Traitorous most commonly refers to disloyalty to a government or nation: a traitorous double agent.Treacherous suggests a propensity for betraying trust or faith: “She gave the treacherous impulse time to subside” (Henry James). Perfidious suggests vileness of behavior and often deceitfulness: a perfidious assassin.