Merriam-Webster's Collegiate® Dictionary, 11th Edition
REIN
I. noun see: retain
Date: 14th century
a strap fastened to a bit by which a rider or driver controls an animal,
2. a restraining influence ; check , controlling or guiding power, opportunity for unhampered activity or use ,
II. verb
Date: 15th century
transitive verb to control or direct with or as if with reins, to check or stop by or as if by a pull at the reins , intransitive verb to submit to the use of reins, to stop or slow up one's horse or oneself by or as if by pulling the reins