give/allow somebody/something free/full rein

give/allow somebody/something free/full ˈrein

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give/allow free/full ˈrein to somebody/something

not restrict, limit or control something: In a novel the author need not keep to the facts, but a textbook is not the place to give free rein to your imagination. OPPOSITE: keep a tight rein on somebody/something
A rein is a long leather band that is fastened around a horse’s neck and used by the rider to control the speed of the horse.