MX = MEPIS + antiX
The distro got started back in 2014 as a collaboration between the MEPIS and antiX communities.
* M for MEPIS, which was an older Debian based distro whose main development got shut down in 2014, and their community was looking for a new home.
* X for antiX, the lightweight Debian based distro that still exists today in parallel and is also one of the distros that took a stand against age verification.
The antiX community still actively maintains MX Linux to this day. That's why both distros share a lot of tools, for example the MX Tools have antiX roots, and both offer sysvinit as an option, which is something you hardly see in mainstream Debian distros.