The bouquet struck her full in the chest, and a little billet-doux fell out of it into her lap.
-- E. M. Forster, Where Angels Fear to Tread
Or you receive a billet doux in a careless scrawl you can't read. What sort of billet doux is that, I ask you?
-- William H. Gass, Willie Masters' Lonesome Wife
“A billet-doux means love letter, in French like.” “Then why didn't you just say love letter?” “Because French is the language of love, my boy. Something you should keep in mind, but will soon forget.”
-- William W. Johnstone and J. A. Johnstone, The Brother's O'Brien
Billet-doux literally means "sweet note" in French. It entered English in the 1660s.