This idiom describes a person whose qualities are hidden or someone who is little known and becomes successful unexpectedly.
The phrase was originally horse racing jargon. A dark horse was one which wasn’t known to the bettors and was therefore difficult to place odds on. The figurative use later spread to other fields, including politics, and has come to apply to anyone who comes under scrutiny but is previously little known.
The first known mention of the concept is in Benjamin Disraeli’s novel The Young Duke (1831). Disraeli’s protagonist, the Duke of St. James, attends a horse race with a surprise finish: “A dark horse which had never been thought of, and which the careless St. James had never even observed in the list, rushed past the grandstand in sweeping triumph.”
The phrase seems to have first been used figuratively in referring to the candidates for academic preferment:
The Saturday Review, 1860 – “A Headship … often given by the College conclaves to a man who has judiciously kept himself dark.”
Sketches from Cambridge, 1865 – “Every now and then a dark horse is heard of, who is supposed to have done wonders at some obscure small college.”
Politically, the concept came to America in the nineteenth century when it was first applied to James K. Polk, a relatively unknown Tennessee politician who won the Democratic Party’s 1844 presidential nomination over a host of better-known candidates. Polk won the nomination on the ninth ballot at his party’s national nominating convention, and went on to become the country’s eleventh president.
Dark horses aren’t always successful. Perhaps the two most famous unsuccessful dark horse presidential candidates in American history were Democrat William Jennings Bryan, and Republican businessman Wendell Willkie.
The idiom is popular, having been used as book, song, and film titles, as well as for various businesses.
But don’t rely on crossword puzzles to give it the currently accepted meaning. In one of last Sunday’s major crosswords, one of the clues was “dark horse.” I struggled for a long time to guess what three-letter word would work and finally had to look it up. The answer was “bay.” Bay? Sorry, “bay” is only dusky; “black” is dark.