un ‧ faith ‧ ful /ʌnˈfeɪθf ə l/ BrE AmE adjective
[ Word Family: noun : ↑ faith , ↑ faithfulness ≠ ↑ unfaithfulness , ↑ faithful , ↑ faithlessness ; adjective : ↑ faithful ≠ ↑ unfaithful , ↑ faithless ; adverb : ↑ faithfully ≠ ↑ unfaithfully ]
someone who is unfaithful has sex with someone who is not their wife, husband, or usual partner ⇨ infidelity
unfaithful to
Geoff had been unfaithful to her on many occasions.
—unfaithfully adverb
—unfaithfulness noun [uncountable]