The Latin word lumbus, from which we get lumbar, meant the loin, and was usually used in the plural, lumbi. In the 19th century the middle of the back in the region of the loins became known as the lumbar region—hence a lumbar puncture, first performed on dead bodies by Cotugno in the middle of the 18th century but not introduced into clinical practice until 1891, by Quincke.
In Latin the suffix -ago, or -igo, or -ugo was often used to denote a disease, giving us albugo (a white opacification of the cornea), caligo (dim vision), impetigo, intertrigo, lentigo, porrigo (dandruff), prurigo, serpigo, tentigo (priapism), vertigo, and vitiligo, more than half of them diseases of the skin. So lumbago was a pain in the loins, or later in the middle of the back.
As with so many medical words, the Latin had a Greek equivalent, in this case etymologically unrelated to it: ο ' ςφυσ (osphus), from which several diseases derived, now to be found only in 19th century medical dictionaries—osphyalgia (lumbago), osphyarthritis (gout in the loins), osphyocele (a lumbar hernia), and osphyomyelitis (inflammation of the lumbar spinal cord).
Lumbus also meant a cut of meat from the loin. The cut of meat that we know as sirloin has been popularly supposed to have come from “a title given to the loin of beef, which one of our kings knighted in a fit of good humour,” as Doctor Johnson defined one meaning of sir. This conceit has been variously attributed to Henry VIII, James I, and Charles II. But the original spelling was surloin, which shows its true and more prosaic origin—from being the piece of meat that lies in the upper part of the loin. In turn, the double sirloin joined by the lumbar spine became punningly known as a baron of beef.
From lumbulus, the diminutive of lumbus, the French derived the word lumbles, meaning the edible intestines of an animal, usually a deer. Then lumbles or lombles became nombles, which came into English as numbles. And a dish made from the numbles was called numble-pie. Now there was a time when the indefinite article got joined to the words it qualified (such as “anoumpere” and “anaranj,” from “noumpere” and “naranj”); later when the article got separated again a new word was sometimes formed (“an umpire,” “an orange”) (see BMJ 1999;318:1758 and 2000;321:953). So in the 15th century numbles lost its initial “n” and became umbles, possibly also through confusion with the supposed French word l'umbles (from lumbles). So numble-pie became umble-pie (first recorded in Pepys' Diary for 8 July 1663). And by deliberate misassocation with the word humble, and because the umble-pie would have been given to the servants while their lord ate the venison, eating humble pie came to have its modern meaning. Very appropriate, for eating humble pie is after all a sort of backing down.
Footnotes
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