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 minutes. Some American trotters will even exceed this record. The best hackneys are produced from thoroughbred sires mated with halfbred mares. (French, haguenée; 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 hukeneys hackneys and geldynges.” —Froissart.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894