The Collaborative International Dictionary
Billet-doux \Bil`let-doux"\ (b[i^]l`l[asl]*d[=oo]"), n.; pl. Billets-doux (b[i^]l`l[asl]*d[=oo]z"). [F. billet note + doux sweet, L. dulcis.] A love letter or note.
A lover chanting out a billet-doux.
--Spectator.
Douglas Harper's Etymology Dictionary
Wiktionary
n. A love letter.
Usage examples of "billet-doux".
Phoebe pictured Maeve, sitting in regal splendor amid her pillows and her billet-doux, opining on the imbecility of love.
When Obie Yount received his first scented billet-doux, and had Florian translate it, and learned that his admirer was a widow woman, he recoiled.
Wouldn't your hands shake if you'd just got a billet-doux from a love-lorn suitor in the bar?
Well, didn't Napoleon write many a billet-doux to his inamorata from the bloody battlefield, with the dead still strewn outside his candle-lit tent?