ˈhɑː(ɹ).nəs keep in check rule one's temper stop or check by or as if by a pull at the reins He reined in his horses in front of the post office 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 control and direct with or as if by reins rein a horse one of a pair of long straps (usually connected to the bit or the headpiece) used to control a horse any means of control he took up the reins of government