If you say that someone is faithless, you mean that they are disloyal or dishonest. She decided to divorce her increasingly faithless and unreliable husband. someone who is faithless cannot be trusted
deliberately and abominably disloyal or likely to betray trust or confidence; "the faithless Benedict Arnold"; "a lying traitorous insurrectionist
deliberately and abominably disloyal or likely to betray trust or confidence; "the faithless Benedict Arnold"; "a lying traitorous insurrectionist"