rein 
(rān)
(rān)n.
1. Either of two long narrow straps attached to each end of the bit of a bridle and used by a rider or driver to control a horse or other animal: The left rein slipped out of the driver's hands. The rider pulled on the reins, and the horse began to slow down.
2. A means of restraining or checking: kept a tight rein on expenditures.
3. often reins A means of controlling or directing: the reins of government.
v. reined, rein·ing, reins
v.tr.
1. To check or hold back by the use of reins. Often used with in or up: reined in the horse.
2. To restrain or control. Often used with in: "a team of strong personalities who would test the limits of prudence unless kept firmly reined in" (Tim Zimmermann).
v.intr.
Idioms: To control a horse, for example, with reins. Often used with in or up.
draw rein
To stop a horse, for example, by pulling on the reins.
draw in the reins
1. To slow down or stop a horse or other animal by putting pressure on the reins.
2. To restrain or control.
give free/full rein to
To release from restraints; allow to go unchecked: gave free rein to her emotions.
[Middle English, from Old French resne, reine, from Vulgar Latin *retina, from Latin retinēre, to retain; see RETAIN.]