dark horse

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): “ […] and a dark horse, which had never been thought of, […] rushed past the grand stand in sweeping triumph.”[1]

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dark horse (plural dark horses)

1. (idiomatic) Someone who possesses talents or favorable characteristics that are not known or expected by others.
   • 2005, Steve Augarde, Celandine, London: Corgi Books, published 2006, →ISBN, page 13:

     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?”

   • 2009, Sophie Kinsella, Twenties Girl: A Novel, London: Black Swan, published 2010, →ISBN, page 183:

     “Well!” Genevieve laughs – the kind of bright, trilling laugh you give when you’re really quite annoyed about something. “Ed, you are a dark horse! I had no idea you had a girlfriend!”

2. (idiomatic, politics) A candidate for an election who is nominated unexpectedly, without previously having been discussed or considered as a likely choice.
3. (horse racing) A horse whose capabilities are not known.
4. Used other than with a figurative or idiomatic meaning: see dark,‎ horse.

• darkhorse

• black sheep
• underdog

• dark horse on Wikipedia.Wikipedia