The Collaborative International Dictionary of English v.0.48:

Kidney \Kid"ney\ (k[i^]d"n[y^]), n.; pl. Kidneys
   (k[i^]d"n[i^]z). [OE. kidnei, kidnere, from Icel. koi[eth]r
   belly, womb (akin to Goth. gipus, AS. cwi[thorn] womb) + OE.
   nere kidney; akin to D. nier, G. niere, OHG. nioro, Icel.
   n[=y]ra, Dan. nyre, Sw. njure, and probably to Gr. nefro`s
   Cf. Kite belly.]
   [1913 Webster]
   1. (Anat.) A glandular organ which excretes urea and other
      waste products from the animal body; a urinary gland.
      [1913 Webster]

   Note: In man and in other mammals there are two kidneys, one
         on each side of vertebral column in the back part of
         the abdomen, each kidney being connected with the
         bladder by a long tube, the ureter, through which the
         urine is constantly excreted into the bladder to be
         periodically discharged.
         [1913 Webster]

   2. Habit; disposition; sort; kind; as, a man of a different
      kidney. --Shak.
      [1913 Webster +PJC]

            There are in later times other decrees, made by
            popes of another kidney.              --Barrow.
      [1913 Webster]

            Millions in the world of this man's kidney.
                                                  --L'Estrange.
      [1913 Webster]

            Your poets, spendthrifts, and other fools of that
            kidney, pretend, forsooth, to crack their jokes on
            prudence.                             --Burns.
      [1913 Webster]

   Note: This use of the word perhaps arose from the fact that
         the kidneys and the fat about them are an easy test of
         the condition of an animal as to fatness. "Think of
         that, -- a man of my kidney; -- . . . as subject to
         heat as butter." --Shak.
         [1913 Webster]

   3. A waiter. [Old Cant] --Tatler.
      [1913 Webster]

   Floating kidney. See Wandering kidney, under Wandering.


   Kidney bean (Bot.), a sort of bean; -- so named from its
      shape. It is of the genus Phaseolus (Phaseolus
      vulgaris). See under Bean.

   Kidney ore (Min.), a variety of hematite or iron
      sesquioxide, occurring in compact kidney-shaped masses.

   Kidney stone. (Min.) See Nephrite, and Jade.

   Kidney vetch (Bot.), a leguminous herb of Europe and Asia
      (Anthyllis vulneraria), with cloverlike heads of red or
      yellow flowers, once used as a remedy for renal disorders,
      and also to stop the flow of blood from wounds;
      lady's-fingers. Kidney-form

WordNet (r) 3.0 (2006):

kidney
    n 1: either of two bean-shaped excretory organs that filter
         wastes (especially urea) from the blood and excrete them
         and water in urine; "urine passes out of the kidney through
         ureters to the bladder"

Moby Thesaurus II by Grady Ward, 1.0:

135 Moby Thesaurus words for "kidney":
   abatis, abdomen, animus, anus, appendix, aptitude, bent, bias,
   blind gut, blood, bowels, brain, brains, brand, breed, cast, cecum,
   character, chitterlings, clan, cockscomb, colon, color,
   constitution, denomination, description, designation, diathesis,
   disposition, duodenum, eccentricity, endocardium, entrails,
   feather, foregut, form, genre, genus, giblets, gizzard,
   glomerulonephritis, grain, guts, haslet, heart, hindgut,
   idiosyncrasy, ilk, inclination, individualism, innards,
   inner mechanism, insides, internals, intestine, inwards, jejunum,
   kidney stone, kidneys, kin, kind, kishkes, label, large intestine,
   leaning, line, liver, liver and lights, lot, lung, make, makeup,
   manner, mark, marrow, mental set, mettle, midgut, mind, mind-set,
   mold, nature, nephritis, nephrolithosis, nephrosis, number,
   perineum, persuasion, phylum, predilection, predisposition,
   preference, proclivity, propensity, pump, pylorus, race, rectum,
   renal calculus, renal hematuria, set, shape, slant,
   small intestine, sort, species, spleen, stamp, stomach, strain,
   streak, stripe, style, sweetbread, temper, temperament, tendency,
   the like of, the likes of, ticker, tongue, tribe, tripe, tripes,
   turn, turn of mind, twist, type, uremia, variety,
   vermiform appendix, viscera, vitals, warp, works