E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Dark Horse (A).
A racing term for a horse of good pretensions, but of which nothing is positively known by the general public. Its merits are kept dark from betters and book-makers.
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“At last a Liberal candidate has entered the field at Croydon. The Conservatives have kept their candidate back, as a dark horse.”—Newspaper paragraph, January, 1886.
