From Classic Encyclopedia 1911
HACKNEY (from Fr. haquenee, Lat. equus, an ambling horse or mare, especially for ladies to ride; the English "hack" is simply an abbreviation), originally a riding-horse. At the present day, however, the hackney (as opposed to a thoroughbred) is bred for driving as well as riding (see Horse: Breeds). From the hiring-out of hackneys, the word came to be associated with employment for hire (so "a hack," as a general term for "drudge"), especially in combination, e.g. hackney-chair, hackney-coach, hackney-boat. The hackney-coach, a coach with four wheels and two horses, was a form of hired public conveyance (see Carriage).