1. A horse, hackneyed or let out for common hire; also, a horse used in
all kinds of work, or a saddle horse, as distinguished from hunting and
carriage horses.
2. A coach or carriage let for hire; particularly, a coach with two
seats inside facing each other; a hackney coach.
On horse, on foot, in hacks and gilded chariots.
Pope.
3. A bookmaker who hires himself out for any sort of literary work; an
overworked man; a drudge.
Here lies poor Ned Purdon, from misery freed, Who long was a
bookseller's hack.
Goldsmith.