noun, na-adjective, noun which may take the genitive case particle 'no'
unfaithfulness; infidelity; unchastity
Refers to sexual unfaithfulness in a committed relationship, especially marital infidelity. Often used in formal or literary contexts.
彼の不貞が原因で離婚した。
They divorced because of his infidelity.
Unfaithful acts are unforgivable.
不貞 means 'infidelity; unfaithfulness,' the direct opposite of the moral constancy or chastity expressed by 貞.
不貞 means infidelity or unchastity, the direct opposite of the faithfulness described by 操（みさお）.
Compound of 不 (fu, 'not') and 貞 (tei, 'chastity, fidelity'). The kanji 貞 originally means 'upright' or 'faithful', so 不貞 literally means 'not faithful'.