rein (n.) Look up rein at Dictionary.com
c.1300, "strap fastened to a bridle," from Old French rene, probably from Vulgar Latin *retina "a bond, check," back-formation from Latin retinere "hold back" (see retain). The verb is c.1300, from the noun. Figurative extension "put a check on" first recorded 1580s. Related: Reined; reining.