dark horse

noun

Etymology: Originally an allusion to an unknown horse with a dark coat winning a race, as used in the 1831 novel The Young Duke by Benjamin Disraeli (1804–1881).

1. Someone who possesses talents or favorable characteristics that are not known or expected by others.

   “‘She’s a dark horse,’ he said. ‘She knows just as much about climbing mountains as you or I. In fact, she was ahead of me the whole time, and I lost her.’”

   “As she pulled the door closed behind her, she heard the nurse say, “Well! You’re a dark horse, I must say! Do you know that extraordinary-looking girl?””

2. A candidate for an election who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.

   “That left Cardinal Robert Francis Prevost, 69 — a quiet American dark horse who had, surprisingly, emerged in the evening’s vote — as a source of particular interest.”

3. A horse whose capabilities are not known.
4. Used other than figuratively or idiomatically: see dark, horse.