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To tighten control over someone or something; to restrain, slow down, or diminish someone or something. A noun or pronoun can be used between "rein" and "in." You need to rein in your journalists. They can't be writing such inflammatory pieces. I'm trying to rein in my enthusiasm a bit, because I don't want to be disappointed.
rein someone or something in
to bring someone or something under control; to slow down someone or something. Fred is getting out of hand. The boss undertook to rein him in a bit. The boss is trying to rein in Jane's enthusiasm.
1. To restrain or control something or someone: The coalition tried to rein in its more militant members. I reined my anger in and refused to fight.
2. To make a horse move more slowly or stop by pulling back on reins: Rein in your horse while this truck goes by. The horses wanted to break free, but the rancher reined them in.