The Collaborative International Dictionary of English v.0.48:

Rein \Rein\, v. t. [imp. & p. p. Reined (r?nd); p. pr. & vb.
   n. Reining.]
   1. To govern or direct with the reins; as, to rein a horse
      one way or another.
      [1913 Webster]

            He mounts and reins his horse.        --Chapman.
      [1913 Webster]

   2. To restrain; to control; to check.
      [1913 Webster]

            Being once chafed, he can not
            Be reined again to temperance.        --Shak.
      [1913 Webster]

   To rein in or To rein up,
      (a) to check the speed of, or cause to stop, by drawing
          the reins. Hence,
      (a) to cause (a person) to slow down or cease some
          activity; -- to rein in is used commonly of superiors
          in a chain of command, ordering a subordinate to
          moderate or cease some activity deemed excessive.
          [1913 Webster +PJC]