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A little known, unexpectedly successful entrant, as in You never can tell? some 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.
American Heritage Idioms
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Phrase(s): dark horse
Fig. someone or something whose abilities, plans, or feelings are little known to others. (From a race horse about which little or nothing is known.) • It’s difficult to predict who will win the prize—there are two or three dark horses in the tournament. • Everyone was surprised at the results of the election. The dark horse won.
McGrawhill's American Idioms And Phrasal Verbs