#1 rein in v. stop or slow up one's horse or oneself by or as if by pulling the reins; "They reined in in front of the post office". wn #2 rein in v. control and direct with or as if by reins; "rein a horse". wn #3 rein in v. stop or check by or as if by a pull at the reins; "He reined in his horses in front of the post office". wn