[1] : 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
see also:
reined reining to rein in to rein up 

Rein >> R Words
Rein, definition of term: Rein
rein_pag1.html

Rein In >> R Words
Rein In, definition of term: Rein In
rein+in_pag1.html