"small saddle horse let out for hire," c. 1300, from place name Hackney (late 12c.), Old English Hacan ieg "Haca's Isle" (or possibly "Hook Island"), the "isle" element here meaning dry land in a marsh. Now well within London, it once was pastoral and horses apparently were kept there. Hence the use for riding horses, with subsequent deterioration of sense (see hack (n.2)). Old French haquenée "ambling nag" is an English loan-word.
"trite, so overused as to have become uninteresting," 1749, figurative use of past-participle adjective from hackney (v.) "use a horse for riding" (1570s), hence "make common by indiscriminate use" (1590s), from hackney (n.), and compare hack (n.2) in its specialized sense of "one who writes anything for hire." From 1769 as "kept for hire."
"person hired to do routine work," c. 1700, ultimately short for hackney "an ordinary horse, horse for general service (especially for driving or riding, as opposed to war, hunting, or hauling)," c. 1300. This word is probably from the place name Hackney, Middlesex. Apparently nags were raised on the pastureland there in early medieval times. Extended sense of "horse for hire" (late 14c.) led naturally to "broken-down nag," and also "prostitute" (1570s) and "a drudge" (1540s), especially a literary one, one who writes according to direction or demand. Sense of "carriage for hire" (1704) led to modern slang for "taxicab." As an adjective, 1734, from the noun. Hack writer is first recorded 1826, though hackney writer is at least 50 years earlier. Hack-work is recorded from 1851.
HACK. A hackney coach. The term hack is also frequently applied by women to any article of dress, as a bonnet, shawl, &c., which is kept for every day use. [Bartlett, "Dictionary of Americanisms," 1848]