rein (n.) Look up rein at Dictionary.com
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.
rein (v.) Look up rein at Dictionary.com
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.