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The term "rein" primarily refers to a strap, usually used in pairs, that is attached to a bit in an animal's mouth and used by a rider or driver to control the animal's direction and speed. This equestrian use has led to figurative meanings, where "reins" can represent a restraining influence, controlling power, or the opportunity for unhampered activity.1

The verb "rein" means to control or direct something or someone, or to check or stop an animal by pulling on the reins. The phrase "rein in" specifically means to limit or control something, or to make an animal stop. For example, one might "rein in" spending or "rein in" impatience. The word originates from Middle English and has been in use since the 13th century.231

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