also billet doux, 1670s, "short love letter," French, literally "sweet note," from billet "document, note" (14c., diminutive of bille "a writing, a list, a seal;" see bill (n.1)) + doux "sweet," from Latin dulcis (see dulcet).
"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).
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. Meaning "paper intended to give public notice of something, exhibited in a public place" is from late 15c. Sense of "paper money, bank-note" is from 1660s. 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-).
<a href="https://www.etymonline.com/word/billet-doux">Etymology of billet-doux by etymonline</a>
Harper, D. (n.d.). Etymology of billet-doux. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/billet-doux