unfaithful
un-faith-ful
adjective
Not loyal or true, especially to a spouse, partner, or promise.
"She discovered her husband had been unfaithful."
Thesaurus
Synonyms: two-timing, cheating, adulterous, treasonable, disloyal, treasonous, traitorous, faithless, apostate, perfidious, untrue, punic
Antonyms: faithful