Meaning

noun one of a pair of long straps (usually connected to the bit or the headpiece) used to control a horse

noun any means of control

verb control and direct with or as if by reins

originFrom Middle English rein, reyne, from Anglo-Norman reyne, resne, from Early Medieval Latin retina, ultimately from Classical Latin retineō (“hold back”), from re- + teneō (“keep, hold”). Compare modern French rêne. Displaced native Old English brīdel (“bridle, rein”), Old English wealdleþer, ġewealdleþer (“rein, bridle”, literally “control strap”), Old English sāl (“cord, rein”), Old English…

Etymology from Wiktionary, CC BY-SA 3.0