A strap attached to a horse's bit for controlling and guiding the horse; to control or hold back something or someone.
From Old French 'rene,' derived from Latin 'retinere' (to hold back), from 're-' (back) and 'tenere' (to hold). The word came to English through Norman French in the 1200s.
The phrase 'free rein' literally means giving a horse complete control, but we use it to mean allowing freedom of action—the metaphor stuck because horseback riding was so central to medieval life!
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