(n.) a hack journalist, a writer who will take on any assignment
You might have spotted a bit of nineteenth century slang on HH recently: an XYZ was once a hack journalist, who would take on any work they could find so long as they were paid.
If you have the latest HH book, The Cabinet of Linguistic Curiosities, you might have come across the story behind this one already. In the late 1800s, a freelancer posted an add in the back pages of The Times, offering, according to an 1889 Dictionary of Slang, Jargon and Cant, “to perform all descriptions of literary work at very moderate and unprofessional prices.”
The advertisement was placed by a writer known only by the pseudonym “XYZ”—which soon caught the attention of enough people to lodge the name in the Victorian vernacular.