The kidneys, the supposed seat of the desires and affections; used metaphorically for "heart." The "reins" and the "heart" are often mentioned together, as denoting the whole moral constitution of man (Ps. 7:9; 16:7; 26:2; 139:13; Jer. 17:10, etc.).
·noun ·pl The kidneys; also, the region of the kidneys; the loins. II. Reins ·noun ·pl The inward i...
Webster's Dictionary of the English Language
(i.e. kidneys). In the ancient system of physiology the kidneys were believed to be the seat of desi...
William Smith's Bible Dictionary