rein

Concise Oxford English Dictionary © 2008 Oxford University Press:
rein/reɪn/

▶noun

• 1 a long, narrow strap attached at one end to a horse's bit, typically used in pairs to guide or check a horse in riding or driving.

  ■ Brit. a pair of straps used to restrain a young child.

• 2 (reins) the power to direct and control: a new manager will soon take over the reins.

▶verb

• 1 check or guide (a horse) by pulling on its reins.

• 2 (often rein someone/thing in/back) restrain.

– phrases
draw rein Brit. stop one's horse.
(a) free rein freedom of action or expression.
keep a tight rein on exercise strict control over.

– origin ME: from OFr. rene, based on L. retinere ‘retain’.

usage: The phrase a free rein, which derives from the literal meaning of allowing a horse to move freely without being controlled by reins, is sometimes misinterpreted and wrongly written as a free reign.