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see rein (n.). Figurative sense "means of controlling; control, check, restraint" is from early 14c.
c.1300, "strap fastened to a bridle," from Old French rene, resne "reins, bridle strap, laces" (Modern French rêne), probably from Vulgar Latin *retina "a bond, check," back-formation from Latin retinere "hold back" (see retain). To give something free rein is originally of horses.
c.1300, from rein (n.). Figurative extension "put a check on" first recorded 1580s. Related: Reined; reining. To rein up "halt" (1550s) is from the way to make a horse stop by pulling up on the reins.
The kidneys, loins, or lower back.
the kidneys, the supposed seat of the desires and affections; used metaphorically for "heart." The "reins" and the "heart" are often mentioned together, as denoting the whole moral constitution of man (Ps. 7:9; 16:7; 26:2; 139:13; Jer. 17:10, etc.).