Multiple times from multiple people I've heard the M in JAMStack described as standing for Markdown. It's an easy mistake to make, since static site generators have been so closely tied to Markdown for so long. Jekyll, for instance, has supported Markdown as the default way to create content pages going back to some of its earliest versions in 2009 and almost every other major static site generator today does the same. So if you know the history of the tools that the JAMStack is based upon, it makes sense to think the M is for Markdown.
However, the M is actually for markup. Markup is a much broader term that encompasses Markdown, which is a lightweight form of markup. There are other similar types of lightweight markup that some static site generator tools support including AsciiDoc and LaTeX. HTML is also markup (it's in the name after all - Hypertext Markup Language).
JAMStack was a term essentially created by Netlify in conjunction with some other companies beginning to work on the tooling for this approach back around 2016. Having written books and articles and presented on the topic extensively, I was approached about it back then and, to be honest, I didn't love the name at the time, but I understood the reasoning behind it and, truthfully, didn't have any better suggestions.
The problem was that up until then we'd been simply calling them static sites and they were built with static site generators. This didn't reflect the reality, which was that these sites had become increasingly dynamic. Today's JAMStack sites are often indistinguishable from any other dynamic web application incorporating things like dynamic content, authentication, and much more. Calling them static sites would be both unfair and misleading. So JAMStack it is - but let's emphasize the M!