Alexander Dyce, A General Glossary to Shakespeare's Works

break up, to carve,—used metaphorically of opening a letter: “Boyet, you can carve; Break up this capon,” LOVE'S LABOUR'S LOST, iv. 1. 56 ; “An it shall please you to break up this,” THE MERCHANT OF VENICE, ii. 4. 10. On the first of these passages Theobald observes:“Our poet uses this metaphor as the French do their poulet; which signifies both a young fowl and a love-letter. Poulet, amatoriæ literæ, says Richelet; and quotes from Voiture, Repondre au plus obligeant poulet du monde, To reply to the most obliging letter in the world. The Italians use the same manner of expression, when they call a love-epistle una pollicetta [polizzetta] amorosa. I ow'd the hint of this equivocal use of the word to my ingenious friend, Mr. Bishop.” Farmer adds: “Henry IV., consulting with Sully about his marriage, says, ‘My niece of Guise would please me best, notwithstanding the malicious reports that she loves poulets in paper better than in a fricasee.’ A message is called a cold pigeon in the letter [by Laneham] concerning the entertainments at Killingworth Castle.”