Source : Webster's Revised Unabridged Dictionary (1913)

Reins \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.

Source : WordNet®

reins
     n : power to control; "the reins of government"