\Reins\ (r[=a]nz), n. pl. [F. rein, pl. reins, fr. L. ren,
pl. renes.]
1. The kidneys; also, the region of the kidneys; the loins.

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} (Arch.), the parts between the crown and
   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.