What does it mean to be a dark horse?

Dark horse is a term used to describe a contender in a competition who is either not well known or not expected to win, but nonetheless performs exceptionally well. It is typically used in the discussion of sports, politics, or other types of contests to describe an unexpected winner. In politics, the term can also refer to someone who unexpectedly wins a nomination to serve as a compromise candidate between factions.

The term is said to have originated in horse racing to describe a horse about which very little is known, and therefore one with the potential to upset the gambling odds if it were to win. Merriam-Webster notes the first known use of the term as being in 1821. British statesman and novelist Benjamin Disraeli used the expression in that manner in his novel The Young Duke (1831). The term came to be used in other types of contests, including political races. In the United States, James K. Polk, the Democratic Party candidate in the 1844 presidential election, was the first “dark horse” nominee in the history of the U.S. presidency. The party was unable to agree upon a candidate from among the more prominent Democratic presidential contenders, and Polk was selected as a compromise candidate. Although Polk was known in political circles, his name had little public recognition: during the campaign period the opposing Whig Party taunted the Democrats with the cry “Who is James K. Polk?” Polk went on to win the election and served as the 11th president of the United States.