kidney: [14] The origins of kidney are a matter of guesswork rather than certain knowledge. Probably the most widely accepted theory is that the -ey element represents ey, the Middle English word for ‘egg’, in allusion to the shape of the kidneys. The first syllable is more problematical, but one possible source is Old English cwith ‘womb’ or the related Old Norse kvithr ‘belly, womb’, in which case kidney would mean etymologically ‘belly-egg’. => egg
