The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   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.
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            My reins rejoice, when thy lips speak right things.
                                                  --Prov. xxiii.
                                                  16.
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            I am he which searcheth the reins and hearts. --Rev.
                                                  ii. 23.
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   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.
      [1913 Webster]