D defs.my
Entry 2 senses Webster, 1913

Reins

/(rānz)/ · IPA /ɹeɪnz/
01 n. pl. The kidneys; also, the region of the kidneys; the loins.
  1. 1.
    The kidneys; also, the region of the kidneys; the loins.
  2. 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.
Phrases & compounds
Reins of a vault — 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.