means The pair of bean-shaped organs that filter waste from your blood and turn it into urine, while also balancing fluids and blood pressure.

from A Middle English word, 'kidenei,' whose deeper roots are genuinely murky — it may join 'kid' (in an old sense of belly or womb) with 'ey,' an Old English word for egg, picturing the organs as little eggs nestled in the belly. That's a plausible reading rather than a proven one; etymologists admit the first part is uncertain. The plural 'kidneys' simply followed the pair.