rein NOUN 1) a long, narrow strap attached at one end to a horse's bit, used in pairs to guide or check a horse. 2) (reins) the power to direct and control.
VERB 1) check or guide (a horse) by pulling on its reins. 2) (often rein in/back) restrain.
(a) free rein — Cf. ↑free rein
keep a tight rein on — Cf. ↑keep a tight rein on
ORIGIN Old French rene, from Latin retinere 'retain' .

English terms dictionary. 2015.