Definition of dark horse in English:

dark horse

noun

  • A candidate or competitor about whom little is known but who unexpectedly wins or succeeds:

    ‘a Gloucester side that could be a dark horse for the title’
    • ‘You should never judge a man by his instrument, because those classical musicians are dark horses.’
    • ‘The normally media shy Peter Menegazzo has been a dark horse in the cattle industry, but now he's the industry's most powerful player.’
    • ‘You certainly hide your bushels under your lights, you dark horses!’
    • ‘Having attracted a familiarity rating of just over two per cent, it's fair to say that in the eyes of the NSW electorate, Morris Iemma is a dark horse.’
    • ‘The dark horse in this constituency is Cllr Finian McGrath, a school principal, who has worked with Dublin Central veteran independent, Tony Gregory.’
    • ‘But by no means does Moe view her students as dark horses.’

Origin

Early 19th century: originally racing slang.