Rein  —n. (in sing. Or pl.) 1 long narrow strap with each end attached to the bit, used to guide or check a horse etc. 2 similar device used to restrain a child. 3 means of control. —v. 1 check or manage with reins. 2 (foll. By up, back) pull up or back with reins. 3 (foll. By in) hold in as with reins. 4 govern, restrain, control.  give free rein to allow freedom of action or expression. Keep a tight rein on allow little freedom to. [french rene from latin retinere *retain]