Webster's Revised Unabridged Dictionary · 1913

Rein

Rein (r?n) , noun

[French rêne, from (assumed) Late Latin retina, from Latin retinere to hold back. See Retain.]

1.
The strap of a bridle, fastened to the curb or snaffle on each side, by which the rider or driver governs the horse.
This knight laid hold upon his reyne. — Chaucer
2.
Hence, an instrument or means of curbing, restraining, or governing; government; restraint.
Let their eyes rove without rein. — Milton
Collocations (2)
To give rein or To give the rein to , to give license to; to leave withouut restrain.
To take the reins , to take the guidance or government; to assume control.

Rein (r?nd) , transitive verb

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. — Shakespeare
Collocations (1)
To rein in or To rein up , (a) to check the speed of, or cause to stop, by drawing the reins. (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.

Rein , intransitive verb

To be guided by reins. [Rare] — Shakespeare