E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
One who runs with the hare and holds with the hounds. George Savile, Marquis of Halifax, adopted the term in the reign of Charles II. to signify that he was neither an extreme Whig nor an extreme Tory. Dryden was called a trimmer, because he professed attachment to the king, but was the avowed enemy of the Duke of York.