1. Banal; trite; stale. Especially used when referring to a phrase, style of writing, or rhetoric that is commonplace and overused.
2. Having been hired or rented out.
Make yourself comfortable, people, and listen to my tale of horses and drudges.
As early as the 13th century, an area in Middlesex, England, had been known as hakenei. As time went on, the named changed to Hakeney, and eventually into Hackney. Eventually this area was absorbed into the expanding city of London, where it resides today.
In the centuries before London engulfed it Hackney was just a small village in the marshes along the river Lea. Hackney didn't have a lot, but it did have a good supply of pastureland, and pastureland is great for raising horses -- so that's just what the resourceful Hackneyians did. Hackney became known for a specific breed of horse: not a workhorse suitable for the farms, but a lighter, prancing horse, suitable for carrying city folk around the city -- an 'ambling horse'.
Hackney horses were a hit in London, and later in other major cities of Europe*. Hackneys are still a recognized and respected breed today -- but they went through a dark period in the 1300s (and didn't we all?). As the Hackneys rose in popularity, it soon came to pass that people started to turn a buck by renting them out to anyone who wanted to spend an afternoon riding through the park. Soon a Hackney horse wasn't so special -- anyone could have one (at least for the day), and as is the case with so much rental property, many of the horses were overworked and raggedy-looking.
Soon 'hackney' referred to any horse for hire, and later, other sorts of vehicles for hire (a hackney coach or hackney carriage, and eventually the modern black taxis of London). This was the start of a large family of slang; soon prostitutes and common drudges were referred to as hackneys. (Demeaning? You bet!) In the 1700s this was shortened to 'hack', a person that works hard for little reward. This is said to be the source of the modern word 'hacker', but more importantly for this node, it is also the source of 'hack writer'.
This specific brand of hack was a person who would jot out generic, low-quality writings on behalf of whomsoever hired him. The writings of these people were referred to as 'hackneyed'. Eventually hackneyed came to refer to all sorts of low-quality and unimaginative writings, regardless of whether the writer was rented out or not.
* In Old French they were called haquenée, in Old Spanish and Portuguese facanea. For a long time etymologists thought that the English word came from the French, and the first edition of the Oxford English Dictionary reported this to be the case. Nowadays we have evidence that the English word came first, but you will still find some sources that give the original theory.