The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         A lover chanting out a billet-doux.      --Spectator.
   [1913 Webster]