Wild Bill recently wrote a post about hackneyed metaphors, and it sent me to the Oxford English Dictionary online to take a whack or a hack at the etymology.
Most of the action seems to have occurred in the 18th century. A “hackney” is an ordinary, work-a-day horse, and thus comes the small one-horse carriage, hackney, we hear about in such works as the Sherlock Holmes tales.
“Hackneyed” as referring over-used words or metaphors seems to spring up not much later.
“Hack,” as referring to a “hack writer” or any other person who is more or less a worker-for-hire, a worker ready to do a quick job, appeared in the 18th century, too. For a while, it referred to a prostitute, but no more, I think.
Now that I know the term referred to a work-a-day horse, I have slightly more affection for hackneyed metaphors.