E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Not thoroughbred, but nearly so. They make the best roadsters, hunters, and carriage horses; their action is showy, and their pace good. A first-class roadster will trot a mile in 2 1/2 minutes. Some American trotters will even exceed this record. The best hackneys are produced from thorough-bred sires mated with half-bred mares. (French, haquenéc; the Romance word haque = the Latin equus; Spanish, hacanéa.)
In ordinary parlance, a hackney, hackney-horse, or hack, means a horse hacked out for hire. These horses are sometimes vicious private horses sold for hacks, or worn-out coach-horses, and cheap animals with broken wind, broken knees, or some other defect.
The knights are well horsed, and the common people and others on litell hakeneys and geldynges.Froissart.