E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A love-letter. In French, poulet means not only a chicken but also a love-letter, or a sheet of note-paper. Thus Henri IV., consulting with Sully about his marriage, says My niece of Guise would please me best, though report says maliciously that she loves poulets in paper better than in a fricasee.
Boyet . . break up this capon [i.e. open this love-letter].Shakespeare. Loves Labours Lost, iv. 1.