The Collaborative International Dictionary of English v.0.48:

Rein \Rein\ (r?n), n. [F. r[^e]ne, 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.
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            This knight laid hold upon his reyne. --Chaucer.
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   2. Hence, an instrument or means of curbing, restraining, or
      governing; government; restraint. "Let their eyes rove
      without rein." --Milton.
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   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.
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