n. pl. kid·neys
1. Anatomy Either one of a pair of organs in the dorsal region of the vertebrate abdominal cavity, functioning to maintain proper water and electrolyte balance, regulate acid-base concentration, and filter the blood of metabolic wastes, which are then excreted as urine.
2. The kidney of certain animals, eaten as food.
3. An excretory organ of certain invertebrates.
4. Temperament; kind: a person of the same kidney.
[Later Middle English kideney, possibly back-formation (on the model of Middle English ey, egg, and its usual plural eiren, eggs, probably influenced by the similarity of the rounded shape of kidneys to that of eggs) from Middle English kideneiren, plural of kideneire, kidney : kide-, perhaps meaning “belly” or “pod, capsule” (in reference to the renal capsule, a tough fibrous membrane surrounding the kidney and surrounded by a layer of fat; compare Modern English dialectal (southern England) kid, pod, peasecod, probably from Old English *cyde, and akin to Old English codd, bag, sack, peasecod) + neire, kidney (from Old English *nēore, from Germanic *neurōn-; akin to Greek nephros, kidney, and Latin (colloquial dialect of the ancient city of Praeneste, east of Rome) nefrōnēs, testicles).]
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