The kidneys are vital organs that essentially act as the body’s filtration system, ensuring that toxins are removed from the blood. They are thus responsible for filtering waste products and other excess substances from the blood, which are then expelled in urine. Additionally, the kidneys play a crucial role in maintaining a balance of electrolytes, regulating blood pressure, and controlling red blood cell production through the secretion of hormones, thus making them crucial overall health and function of both man and animal. One of the oft-repeated instructions for ritual sacrifices given in the Book of Leviticus is that the kilayot (“kidneys”) of an animal sacrifice must be burnt on the altar along with various fats from the animal’s body. In fact, of the thirty-one times that the word kilayot appears in the entire Bible, fifteen of those instances refer to this specific rule (Lev. 3:4, 3:10, 2:15, 4:9, 4:7, 8:16, 8:25, 9:10, 9:19). In the essay before you, we explore the etymology of the word kilayot, and compare it to its two possible synonyms in Biblical Hebrew — atzeh and tuchot.

In addition to the word kilayot in the Bible referring to “kidneys” in the anatomical sense, the word is also used in a more abstract sense to refer to one’s innermost thoughts and motives. It is in this cognitive sense that Hashem is said to have the ability to examine man’s heart and kidneys (Jer. 11:20, 17:10, 20:12, Ps. 7:10). In such contexts, the term kilayot is often translated into English as “reins” (which is related to the English words renal and adrenaline).

Besides for the aforementioned instances of kilayot in the Bible, this word also appears in the Mishnah (Chullin 3:2, 4:1, Tamid 4:3). Interestingly, the word kilayot always appears in the Bible and Mishnah in plural form, although theoretically, its singular form should be kilyah. In the Talmud, the Hebrew/Aramaic word kulya refers to “a single kidney” (Pesachim 64b, Chullin 93a, 97a, 126b, 128b–129a, Bechorot 39a, Kritot 14a, 23a).

Rabbi Moshe Tedeschi Ashkenazi (1821–1898) in his work Otzar Nirdafim (§344) connects the word kilayot to the word kele (“jail, incarceration”), spelled with a final ALEPH. He accounts for this connection by explaining that as internal organs, the kidneys are embedded deep within a person’s body, as though they are “hidden” or “jailed’ inside. Indeed, Ibn Ezra (to Ps. 7:10) understands that the term kilayot can sometimes be meant metaphorically as a reference to the thoughts and beliefs “hidden” deep within a person’s consciousness.

In a similar vein, Ibn Ezra (to Ex. 23:25, Lev. 3:4 and Ps. 16:7, 139:13, cited by Radak in Sefer HaShorashim) connects these two meanings of the word kilayot by postulating that the bodily kidneys are the seat of libidinous desire, leading him to explain kilayot as cognate........

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