kidney

Etymology

From Middle English kednei, kidenei, from earlier kidnēre, kidenēre (“kidney”), of obscure origin and formation. Probably a compound consisting of Middle English *kid, *quid (“belly, womb”), from Old English cwiþ, cwiþa (“belly, womb, stomach”) + Middle English nēre (“kidney”), from Old English *nēora (“kidney”), from Proto-West Germanic *neurō, from Proto-Germanic *neurô (“kidney”), from Proto-Indo-European *negʷʰr- (“kidney”). If so, then related to Scots nere, neir (“kidney”), Saterland Frisian Njuure (“kidney”), Dutch nier (“kidney”), German Niere (“kidney”), Danish nyre (“kidney”), Norwegian nyre (“kidney”), Swedish njure (“kidney”), Ancient Greek νεφρός (nephrós). Alternate etymology traces the first element to Old English cēod, codd (“sack, scrotum”), from Proto-Germanic *keudō (“sack”) as the terms for testicle and kidney were often interchangeable in Germanic (compare Old High German nioro (“kidney", also "testicle”), Old Swedish vig-niauri (“testicle”). More at codpiece.

noun

1. An organ in the body that filters the blood, producing urine.
   An artificial kidney these days still means a refrigerator-sized dialysis machine. Such devices mimic the way real kidneys cleanse blood and eject impurities and surplus water as urine. 2013-06-01, “A better waterworks”, in The Economist, volume 407, number 8838, page 5 (Technology Quarterly)
2. This organ (of an animal) cooked as food.
3. (figurative, dated) Constitution, temperament, nature, type, character, disposition. (usually used of people)
   30th June, 1788, Robert Burns, letter to Mr Robert Ainslie Your poets, spendthrifts, and other fools of that kidney, pretend, forsooth, to crack their jokes on prudence.
   I shall not want Honour in Heaven For I shall meet Sir Philip Sidney And have talk with Coriolanus And other heroes of that kidney. 1920, T.S. Eliot, A Cooking Egg
4. (obsolete, slang) A waiter.
   I once more desire my readers to consider that as I cannot keep an ingenious man to go daily to Will's under twopence each day merely for his charges, to White's under sixpence, nor to the Grecian without allowing him some plain Spanish, to be as able as others at the learned table; and that a good observer cannot speak with even Kidney at St. James's without clean linen; […] 1709, Richard Steele, The Tatler, volume 1