Wordorigins.org

How did a word for a taxi also become a term meaning overused and worn out?

Hackney comes from the Old French haquenée, meaning a gentle, riding horse, an ambling horse. It was adopted into English in the 14th century meaning a horse of middle size or fair quality. From The Romance of Sir Beues of Hamtoun, a 14th century poem:

Ac nim a ligter hakenai & lef her the swerd Morgelai.
(But take a lighter hackney & leave here the sword Morgelai.)

Very early on, by 1393 at the latest, the word had also acquired the meaning of a horse for hire. From William Langland’s Piers Plowman (C Text) of that year:

Ac hakeneyes hadde thei none. bote hakeneyes to hyre.
(But hackneys had they none, hackneys for hire to boot.)