Rein (r?n), n. [F. rêne, fr. (assumed) LL. retina, fr. L. 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.
To give rein, 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.