illustration from Le Livre de chasse, que fist le comte Febus de Foys, seigneur de Bearn (1375-1400)—source: Gallica – Bibliothèque nationale de France
The French invariable masculine noun sot-l’y-laisse translates in English as oyster in the sense of an oyster-shaped piece of meat contained in a depression on each side of the pelvic bone of a fowl—also read merrythought.
The literal sense of the French word is (the) fool leaves it there, because those bits of meat are at the same time ‘hidden’ in hollows and considered delicious.
It was first recorded, in a handwritten collection of French-Canadian words and expressions, by the Belgian Jesuit and missionary Pierre-Philippe Potier (1708-81) between 1743 and 1758, while he was in Canada (which at that time included Detroit). The word, apparently phonetically transcribed, appears in the plural form:
(from Les écrits de Pierre Potier (Les Presses de l’Université d’Ottawa, 1996), by Pierre-Philippe Potier, Robert Toupin, Pierrette L. Lagarde)
Solilesses M. d’un d’inde & 2 morceaux de viande sur la carcasse * Mʳ gral * stulti relinquunt¹
Solilesses masculine, of a turkey, etc., two morsels of meat on the carcass * Mister the General (?) * stulti relinquunt¹
(¹ Latin stulti relinquunt: the fools leave (it))
The word was recorded in the 5th edition (1798) of the Dictionnaire de l’Académie française:
Sot-l’y-laisse. On appelle ainsi Un morceau très-délicat qui se trouve au-dessus du croupion d’une volaille.
Sot-l’y-laisse. One thus calls a very delicate morsel which is above the rump of a fowl.
Four centuries earlier, from 1387 to 1389, Gaston Phébus (1331-91), count of Foix and viscount of Béarn, had composed Livre de chasse, a book on hunting in which the chapter giving advice on how to cut up a stag contains:
(edited by Joseph Lavallée – Paris, 1854)
le colier² que aucuns apellent fol³ l’i laisse
the collar, which some call (the) fool leaves it there
(² colier: the flesh between the shoulders and the breast)
(³ fol: former form of the masculine fou – the feminine is folle – fol is the origin of English fool.)