means One of the pair of bean-shaped organs that filter waste and excess water from the blood to produce urine.

from A Middle English word, kidenei, of murky and much-debated origin. The popular guess splits it into something like 'egg' plus 'belly,' picturing the organ as an egg nestled in the abdomen — but scholars genuinely disagree, and the first part remains unexplained. The bean shape gave us 'kidney bean,' and the figurative sense of 'temperament' (as in 'a man of that kidney') comes from an old belief that the kidneys were a seat of emotion and disposition.