Rein, v. t. [imp. & p. p. Reined p. pr. & vb. n. Reining.]
 1. To govern or direct with the reins; as, to rein a horse one way or another.
    He mounts and reins his horse.   --Chapman.
 2. To restrain; to control; to check.
 Being once chafed, he can not
 Be reined again to temperance.   --Shak.
 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.