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).
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.”