The Collaborative International Dictionary
Rein \Rein\, v. t. [imp. & p. p. Reined (r?nd); p. pr. & vb. n. Reining.]
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To govern or direct with the reins; as, to rein a horse one way or another.
He mounts and reins his horse.
--Chapman. -
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.
Usage examples of "to rein up".
Once he started to rein up to feed them, but Tarrant waved angrily for him to continue.
A series of dull thuds, followed by screaming from downed horses and yells as riders tried to rein up and avoid becoming entangled in the mass of fallen mounts and unhorsed men.
Often he was compelled to rein up suddenly lest he ride over the Southern lads themselves.
Our troops began to rein up, and I saw the Red Tiger himself leap from his saddle to run up the stairs at the two sentries stationed there.
Tyrion hacked at the man's head as he flashed by, and by then it was too late to rein up.