kidnēre

Kidney appears in Middle English around the 14th century as kidnēre or kidnei. Its origin is one of the genuine mysteries of English etymology. The word has no obvious cognate in any other Germanic language. Old English used the word lungen-ādl for kidney disease and seems to have lacked a specific term for the organ itself. The first element, kid-, might be related to Old English cwith (womb) or Middle English kide (belly), suggesting a compound meaning 'belly-egg' or 'womb-egg,' but this is speculative.

Other Germanic languages use different words entirely: German Niere, Dutch nier, Old Norse nýra — all from Proto-Germanic *neurō, which gave English the now-obsolete 'neer.' The word kidney arrived from somewhere and displaced the Germanic word. Where it came from remains unclear. Some scholars have proposed a compound of Middle English ey (egg) — the kidney's shape — with a prefix of uncertain origin. The egg theory is plausible: the kidney is roughly egg-shaped.

The organ itself was well known to ancient physicians. Egyptian, Greek, and Roman medicine all described the kidneys and their function in producing urine. Galen located them correctly and understood their filtration role, though he believed they attracted urine by a 'specific faculty' rather than by blood filtration. The Latin word ren (plural renes) gives English 'renal.' The Greek nephros gives 'nephrology.' English 'kidney' gives neither. It is an etymological orphan.

The word acquired figurative meaning by the 16th century. 'A man of that kidney' meant a man of that type or disposition — as if the kidneys determined character. Shakespeare used it. The metaphor has faded, but the word kidney still appears in phrases like 'of the same kidney,' meaning of the same sort. The organ that filters waste became a metaphor for sorting people.