Dark Horse

Began as horse racing slang for a horse that gamblers knew nothing about, making it genuinely hard to set odds on. The concept's first known appearance in print is Benjamin Disraeli's 1831 novel The Young Duke: "A dark horse, which had never been thought of... rushed past the grand stand in sweeping triumph." Some etymologists trace an earlier, unattributed use of the term to 1821. From racing, the phrase spread naturally to any contest where a little-known entrant suddenly does well, including politics: James K. Polk, in 1844, is generally considered the first "dark horse" candidate for the American presidency.