Boyd’s Bible Dictionary:
(kidneys). Once believed to be the seat of emotions; hence coupled with the heart (Psa. 7:9; 16:7; Jer. 17:10; 20:12).
“Kidneys” From Concise Bible Dictionary:
Bovine Kidneys
These, “with the fat thereof,” were often burned on the altar in the sacrifices (Lev. 3:4-15; Lev. 4:9; Lev. 7:4, and more. The same word, Kelayoth is translated “reins,” (which signifies “kidneys”) when used symbolically of the inward feelings and affections.
“Reins” From Concise Bible Dictionary:
Human Kidneys
The kidneys, used symbolically for the inward thoughts and feelings (Psa. 7:9; Psa. 16:7; Psa. 26:2; Psa. 139:13; Prov. 23:16; Jer. 12:2; Jer. 17:10; Jer. 20:12; Lam. 3:13; Rev. 2:23). The word translated “reins” in Isaiah 11:5 is elsewhere translated “loins.”
Strong’s Dictionary of Hebrew Words:
Meaning:
feminine of 3627 (only in the plural); a kidney (as an essential organ); figuratively, the mind (as the interior self)
KJV Usage:
kidneys, reins