Reins

Easton's Bible Dictionary (1897)

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.).

Smith's Bible Dictionary (1863)

(i.e. kidneys). In the ancient system of physiology the kidneys were believed to be the seat of desire and longing, which accounts for their often being coupled with the heart. (Psalms 7:9; 26:2; Jeremiah 11:20; 17:10), etc.

Public domain dictionaries.