Source : Webster's Revised Unabridged Dictionary (1913)

Billet-doux \Bil`let-doux"\, n.; pl. {Billets-doux}. [F. billet
   note + doux sweet, L. dulcis.]
   A love letter or note.

         A lover chanting out a billet-doux.      --Spectator.

Source : WordNet®

billets-doux
     See {billet-doux}