“From the same to the same,” said M. Gillenormand, bursting with laughter. “I know what it is. A billet-doux.”
“Ah! let us read it!” said the aunt.
Victor Hugo. Les Misérables
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Long and earnestly did I continue the investigation: but the contemptible reward of my industry and perseverance proved to be only a set of false teeth, two pair of hips, an eye, and a bundle of billets-doux from Mr. Windenough to my wife. I might as well here observe that this confirmation of my lady’s partiality for Mr. W. occasioned me little uneasiness.
Edgar Allan Poe. The Works of Edgar Allan Poe — Volume 4: Loss of Breath (1832)
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“Je croyais que c’était un bill— not a billet doux.” Then, entering the sunny room, to Lolita: “Bonjour, mon petit.”