billet-doux

[bil-ey-doo, bil-ee-; French bee-yey-doo] /ˈbɪl eɪˈdu, ˈbɪl i-; French bi yeɪˈdu/
noun, plural billets-doux
[bil-ey-dooz, bil-ee-; French bee-yey-doo] /ˈbɪl eɪˈduz, ˈbɪl i-; French bi yeɪˈdu/ (Show IPA)
1.
a love letter.
Origin
1665-75; < French: literally, sweet note. See billet1, douce
British Dictionary definitions for billet-doux

billet-doux

/ˌbɪlɪˈduː; French bijɛdu/
noun (pl) billets-doux (ˌbɪlɪˈduːz; French) (bijɛdu)
1.
(old-fashioned or jocular) a love letter
Word Origin
C17: from French, literally: a sweet letter, from billet (see billet1) + doux sweet, from Latin dulcis
Word Origin and History for billet-doux
n.

also billet doux, 1670s, "love letter," French, literally "sweet note," from billet "document, note" (14c., diminutive of bille; see bill (n.1)) + doux "sweet," from Latin dulcis (see dulcet).