noun, plural kidneys.
Anatomy. either of a pair of bean-shaped organs in the back part of the abdominal cavity that form and excrete urine, regulate fluid and electrolyte balance, and act as endocrine glands.
Zoology. a corresponding organ in other vertebrate animals or an organ of like function in invertebrates.
the meat of an animal's kidney used as food.
constitution or temperament: He was a quiet child, of a different kidney from his boisterous brothers.
kind, sort, or class: He is only at ease with men of his own kidney.

1275–1325; Middle English kidenei, kidenere (singular), kideneres, kideneren (plural); origin uncertain; perhaps a compound based either on nere (singular), neres (plural) kidney (Old English *nēore; compare Old High German nioro, Old Norse nȳra); or ei (singular), eiren (plural) egg1, Old English ǣg (singular), ǣgru (plural) (by association with the organ's shape); for the first element compare dial. kid pod (akin to cod2)

kidneylike, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
kidney (ˈkɪdnɪ)
1.  either of two bean-shaped organs at the back of the abdominal cavity in man, one on each side of the spinal column. They maintain water and electrolyte balance and filter waste products from the blood, which are excreted as urineRelated: nephritic, renal
2.  the corresponding organ in other animals
3.  the kidneys of certain animals used as food
4.  class, type, or disposition (esp in the phrases of the same or a different kidney)
Related: nephritic, renal
[C14: of uncertain origin]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

earlly 14c., of unknown origin, originally kidenere, perhaps a compound of O.E. cwið "womb" + ey "egg," in reference to the shape of the organ. Fig. sense of "temperament" is from 1550s. Kidney bean is from 1540s.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

kidney kid·ney (kĭd'nē)
n. pl. kid·neys
Either 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.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
kidney   (kĭd'nē)  Pronunciation Key 

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Either of a pair of organs that are located in the rear of the abdominal cavity in vertebrates. The kidneys regulate fluid balance in the body and filter out wastes from the blood in the form of urine. The functional unit of the kidney is the nephron. Wastes filtered from the blood by the nephrons drain into the ureters, muscular tubes that connect each kidney to the bladder. See also nephron.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

kidneys definition

A pair of organs, the principal parts of the excretory system, located above the waistline at the back of the abdominal cavity. The kidneys filter waste materials from the blood, excreting these wastes in the form of urine; they also regulate the amounts of water and other chemicals in body fluids.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Her kidneys were failing and she had gained a grotesque amount of weight due to
  fluid retention and so forth.
In healthy birds, uric acid is excreted by the kidneys and is seen as the white
  material in their droppings.
Diuretics cause the kidneys to remove sodium and water from the body, thereby
  alleviating pressure on the blood vessel walls.
Finally, there is the liver, which turns fat-soluble poisons into watersoluble
  wastes that can be flushed out through our kidneys.
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