Definition of "dark horse" [dark horse]

• One who achieves unexpected support and success as a political candidate, typically during a party's convention. (noun)
• A little-known, unexpectedly successful entrant, as in a horserace. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Curley’s old mouth pouted contemptuously as she considered them: John with his irascibility, his pomposity and his moments of sheer obstinacy; Paul lathering and shouting and making an exhibition of himself; and now the dark horse Mike, who had never really wanted anything before."