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billet-doux \bil-ay-DOO\, noun;
plural billets-doux \bil-ay-DOO(Z)\:
A love letter or note.

Perhaps she just looked first into the bouquet, to see whether there was a billet-doux hidden among the flowers; but there was no letter.
--William Makepeace Thackeray, Vanity Fair

Young lovers in Victorian England, forbidden to express their affection in public and fearful that strict parents would intercept their billets-doux, sent coded messages through the personal columns in newspapers.
--Susan Adams, "I've got a secret,"Forbes, September 20, 1999