In French, billet doux means "sweet letter." English writers first fell in love with the word during the 17th century and have been committed to using it as a romantic alternative to "love letter" ever since.
French billet doux, literally, sweet letter
1673, in the meaning defined above
👨🏻🏫 Mr. Ng 韋氏詞典 📚 – mw.mister5️⃣.net切換為繁體中文