kidney

From Middle English kideney, kydeney, alteration of earlier kidenere (“kidney”) after ey (“egg”), of obscure origin and formation. Probably a compound consisting of Middle English *kidde (“sack”) + Middle English nere (“kidney”). The first element would be from Old English *cydde (“sack, scrotum”), from Proto-Germanic *kuddijā (“sack”), as the terms for testicle and kidney were often interchangeable in Germanic (compare Old High German nioro (“kidney", also "testicle”), Old Swedish vig-niauri (“testicle”)); the second is from Old English *nēora or Middle Low German nêre (“kidney”), from Proto-West Germanic *neurō, from Proto-Germanic *neurô (“kidney”), from Proto-Indo-European *negʷʰr- (“kidney”). If so, then related to dialectal English near (“kidney”), Scots neir (“kidney”), Saterland Frisian Njuure (“kidney”), West Frisian nier (“kidney”), Dutch nier (“kidney”), German Niere (“kidney”), Danish nyre (“kidney”), Norwegian nyre (“kidney”), Swedish njure (“kidney”), Ancient Greek νεφρός (nephrós, “kidney”).

An alternate etymology traces the first element to Old English cwiþ, cwiþa (“belly, womb, stomach”), though the loss of /w/ in a stressed syllable is difficult to explain.