billet-doux n.

[written statement] late 14c., "formal document; formal plea or charge (in a court of law); personal letter," from Anglo-French bille, Anglo-Latin billa "a writing, a list, a seal," from Medieval Latin bulla "decree, seal, sealed document," in classical Latin "bubble, boss, stud, amulet for the neck" (hence "seal"); see bull (n.2).

The sense of "written statement detailing articles sold or services rendered by one person to another" is from c. 1400; that of "order addressed to one person to pay another" is from 1570s. The meaning "paper intended to give public notice of something, exhibited in a public place" is from late 15c. The sense of "paper money, bank-note" is from 1660s. The meaning "draft of a proposed statute presented to a legislature" is from 1510s.

"sweet to the senses," especially of taste, also "melodious, harmonious," late 14c., doucet, from Old French doucet, diminutive of doux "sweet," earlier dulz, from Latin dulcis, from Proto-Italic *dulkwi-, from PIE *dlk-wi-, suffixed form of root *dlk-u- "sweet," which probably is also the source of Greek glykys "sweet" (compare gluco-).