disloyal, treacherous, or insincere

she felt that to sell the house would be unfaithful to her parents' memory

engaging in sexual relations with a person other than one's spouse or partner, in contravention of a previous promise or understanding

you haven't been unfaithful to him, have you?

her unfaithful husband

not following the facts or the original accurately

dialect poems are often unfaithful in reproduction and unpoetic