Let us suppose that you are a great Anglo-Norman baron of, say, the 13th century. You go out to hunt deer on your estate, taking with you a considerable party of friends, guests, and retainers. After you have shot a few deer, you return and the retainers butcher the animals for use. The better cuts belong to you, the lord of the manor; the rest—head, skin, shoulders,chine, and the edible viscera—are left for the gamekeeper, the huntsmen, and the servants.
Those edible viscera—liver, heart, kidneys, and so forth—were known in Norman French as nombles, a word the French had derived from Latin lumbulus, denoting a cut of meat, from lumbus, "loin." By the 14th century, nombles had passed into English as numbles, by the 15th it was umbles, and by the 16th humbles. The usual way to prepare the humbles was to make them into a meat pie—really a thick rich stew with a crust. So while the gentry were feasting on prime cuts of venison, the gamekeeper and other servants were dining on humble pie.
Today's figurative humble pie appeared about 1830, when it turned up in a book on the vocabulary of East Anglia. It was used in the phrase "make one eat humble pie," which was defined as "to make him lower his tone, and be submissive." The compiler of the book suspected that the phrase derived from the pie made of deer umbles, and in that case, he thought, it should be written "umble-pie, the food of inferiors."