The Collaborative International Dictionary of English v.0.48:
Rein \Rein\, v. t. [imp. & p. p. Reined (r?nd); p. pr. & vb.
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.
[1913 Webster +PJC]