Back in my day, "headless" meant a computer without a monitor, for example a server or a PC that had been repurposed to server duty.

We also had the opposite: before Xrandr or Xinerama, there was an older X feature called "Zaphod Mode" that allowed you to drive a graphics card's multiple outputs (on cards that supported such) mapping each to a different X "screen", Zaphod Beeblebrox, of The Hitchhiker's Guide to the Galaxy, had two heads, you see.

"Headless" can also refer to a browser that doesn't render to the screen, useful for automated integration tests.

Yeah I think the Jamstack folks have decided to use headless in an entirely different way. For example search for headless CMS you'll find loads of CMSs that just run on a server and use an API to talk to the client, but they're just web pages.

It just means not WordPress I think in this context.

Then it’s styleless and markupless (that’s not a word, is it?), too.