• IPA: /ɹeɪn/

rein (plural reins)

1. A strap or rope attached to a bridle or bit, used to control a horse, animal or young child.
2. (figurative) An instrument or means of curbing, restraining, or governing.
   • 1667, John Milton, “Book 10”, in Paradise Lost. A Poem Written in Ten Books, London: Printed [by Samuel Simmons], and are to be sold by Peter Parker […] [a]nd by Robert Boulter […] [a]nd Matthias Walker, […], OCLC 228722708 ↗; republished as Paradise Lost in Ten Books: The Text Exactly Reproduced from the First Edition of 1667: […], London: Basil Montagu Pickering […], 1873, OCLC 230729554 ↗:

     Let their eyes rove without rein.

• French: rêne
• German: Zügel
• Italian: redine, briglia
• Portuguese: rédea
• Russian: по́вод
• Spanish: rienda

rein (reins, present participle reining; past and past participle reined)

1. (transitive) To direct or stop a horse by using reins.

   • He mounts and reins his horse.

2. (transitive) To restrain; to control; to check.
   • c. 1608–1609, William Shakespeare, “The Tragedy of Coriolanus”, in Mr. William Shakespeares Comedies, Histories, & Tragedies: Published According to the True Originall Copies (First Folio), London: Printed by Isaac Iaggard, and Ed[ward] Blount, published 1623, OCLC 606515358 ↗, [Act III, scene iii]:

     Being once chafed, he cannot / Be reined again to temperance.

3. (intransitive) To obey directions given with the reins.
   • 2011, Marie Claire Peck, Rocking Horse Ranch (page 40)

     She worked each horse at a walk, trot, and then a canter. The horses reined well and executed stops quickly.

rein (plural reins)

1. (now rare, archaic, chiefly in plural) A kidney.
   • 1603, Michel de Montaigne, chapter 12, in John Florio, transl., The Essayes, […], book II, printed at London: By Val[entine] Simmes for Edward Blount […], OCLC 946730821 ↗:

     a man subject to these like imaginations […] hath often the stone imaginarily, before he have it in his reines […].

   • 1611, King James Bible, Lamentations 3:13:

     He hath caused the arrows of his quiver to enter into my reins.

2. The inward impulses; the affections and passions, formerly supposed to be located in the area of the kidneys.
   • Bible, Proverbs xxiii. 16

     My reins rejoice, when thy lips speak right things.

   • Bible, Revelation ii. 23

     I am he which searcheth the reins and hearts.