unfaithfulness
不貞
解説 Definition
配偶者や恋人への忠誠心の欠如。約束や誓いに対する不忠実。浮気や背信行為。
Unfaithfulness is the act of not being loyal, especially in a romantic relationship. It often means having a sexual or emotional relationship with someone else in secret. The word can also refer more generally to a lack of loyalty or trust.
覚え方のコツ Memory Tip
まず faithful を「信頼を裏切らず、相手に忠実な」として押さえると覚えやすいです。be faithful to ... の形で「〜に誠実である」と使うので、unfaithfulness はその状態が崩れたもの、と考えると意味がまとまります。-ness は性質を名詞化するので、「faithfulでないこと」全体を指す語。恋愛や結婚での裏切り、約束への不誠実さに結びつけて覚えると定着しやすく、単なる “faith（信仰）不足” ではなく “trust を壊す不忠実さ” だと区別すると混同しにくいです。
Start from `faithful`, which is often used for love and marriage, not only for general loyalty. Add `un-`, and think of breaking that faith in a relationship. It often appears near words like `affair`, `marriage`, and `divorce`.
例文
His unfaithfulness ended their marriage.
彼の不貞は彼らの結婚を終わらせました。
She could not forgive his unfaithfulness.
彼女は彼の不貞を許すことができませんでした。
Unfaithfulness is a common cause of divorce.
不貞は離婚の一般的な原因です。
unfaithfulness の類義語・関連語
unfaithfulness の語源・成り立ち Etymology
un-『否定』+ faithful + -ness で、『信義がある状態』の反対。faith は古フランス語経由でラテン語 fides『信頼・誠実』にさかのぼり、fidelity や confide と同根。『信頼を保つ力がないこと』と見ると、不貞の芯が見える。
This word is built in English from un- "not," faithful, and -ness. Faithful comes from faith, which entered English through Old French from Latin fides, meaning "trust" or "faith." From the idea of being full of faith or loyalty, unfaithfulness developed as the state of lacking that trust or loyalty. Related English words include fidelity and confide.
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