What Does “dark horse” mean?
Definition:
The phrase dark horse is an idiomatic expression that refers to a usually little-known person who unexpectedly wins or succeeds, especially in a competition of some sort.
The first known mention of the phrase 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.”
More on this idiom on Wikipedia
Example(s)
Jane turned out to be a dark horse. Although she had suffered from a severe illness this year, she managed to win the race.
This idiom is in the animals category.