Entries linking to reins
c. 1300, reine, "strap of a bridle," attached to it on either side of the head, by which the rider or driver restrains and guides the animal, from Old French rene, resne "reins, bridle strap, laces" (Modern French rêne), probably from Vulgar Latin *retina "a bond, check," a back-formation from Latin retinere "hold back" (see retain). Compare Latin retinaculum "a tether, halter, rein."
The figurative extension of reins to "guidance, means of controlling; control, check, restraint" is by mid-14c. Hence many expressions, originally from horse-management: Hold the reins "wield power" (early 15c.); take the reins "assume the power of guidance or government" (1610s). To give something free rein also is originally of horses; to give (a horse) the reins (1620s) is to allow it free motion.