dark horse

Meanings

1. Someone who possesses talents or favorable characteristics that are not known or expected by others.
2. A candidate for an election who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.
3. A horse whose capabilities are not known.
4. Used other than figuratively or idiomatically: see dark, horse.

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).