dark horse

noun
1.
a racehorse, competitor, etc., about whom little is known or who unexpectedly wins.
2.
a candidate who is unexpectedly nominated at a political convention.

Origin:
1825–35

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
dark horse
 
n
1.  a competitor in a race or contest about whom little is known; an unknown
2.  a person who reveals little about himself or his activities, esp one who has unexpected talents or abilities
3.  (US) politics a candidate who is unexpectedly nominated or elected

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline
Word Origin & History

dark horse
in politics, 1842, an image from horse racing, though why a dark horse would be not expected to do well is uncertain.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Cultural Dictionary

dark horse definition


An unexpected winner. In politics, a dark horse is a candidate for office considered unlikely to receive his or her party's nomination, but who might be nominated if party leaders cannot agree on a better candidate.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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American Heritage
Idioms & Phrases

dark horse

A little known, unexpectedly successful entrant, as in You never can tellsome dark horse may come along and win a Senate seat. This metaphoric expression originally alluded to an unknown horse winning a race and was so used in a novel by Benjamin Disraeli (The Young Duke, 1831). It soon began to be transferred to political candidates, among the first of whom was James K. Polk. He won the 1844 Democratic Presidential nomination on the eighth ballot and went on to win the election.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
Yet improbably, miraculously, the universe-the ultimate dark horse-beat those odds.
Synonyms
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