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 (Psalm 7:9; 16:7; 26:2; 139:13; Jeremiah 17:10, etc.).

Noah Webster's New International Dictionary of the English Language

1. (n. pl.) The kidneys; also, the region of the kidneys; the loins.

2. (n. pl.) The inward impulses; the affections and passions; -- so called because formerly supposed to have their seat in the part of the body where the kidneys are.