2. 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. My reins rejoice, when thy lips speak right things. (Prov. Xxiii. 16) I am he which searcheth the reins and hearts. (rev. Ii. 23) Reins of a vault, the parts between the crown andd the spring or abutment, including, and having especial reference to, the loading or filling behind the shell of the vault. The reins are to a vault nearly what the haunches are to an arch, and when a vault gives way by thrusting outward, it is because its reins are not sufficiently filled up.
Origin: F. Rein, pl. Reins, fr. L. Ren, pl. Renes.