https://arstechnica.com/gadgets/2024/01/the-oldest-known-version-of-ms-doss-predecessor-has-been-discovered-and-uploaded/#:~:text=It%20began%20in%20mid%2D1980,Microsoft%20licensed%20and%20eventually%20purchased.
86-DOS would later be bought by Microsoft and take over the computing world.
This is the best summary I could come up with:
Microsoft’s MS-DOS (and its IBM-branded counterpart, PC DOS) eventually became software juggernauts, powering the vast majority of PCs throughout the '80s and serving as the underpinnings of Windows throughout the '90s.
It began in mid-1980 as QDOS, or "Quick and Dirty Operating System," the work of developer Tim Paterson at a company called Seattle Computer Products (SCP).
According to the version history available on Wikipedia, this build of 86-DOS would date back to roughly August of 1980, shortly after it lost the "QDOS" moniker.
By late 1980, SCP was sharing version 0.3x of the software with Microsoft, and by early 1981, it was being developed as the primary operating system of the then-secret IBM Personal Computer.
By the middle of 1981, roughly a year after 86-DOS began life as QDOS, Microsoft had purchased the software outright and renamed it MS-DOS.
Microsoft also retained the ability to license the software to other computer manufacturers as MS-DOS, which contributed to the rise of a market of mostly interoperable PC clones.
The original article contains 394 words, the summary contains 168 words. Saved 57%. I'm a bot and I'm open source!
I believe Microsoft tactfully and retroactively changed it to "disk" once it went commercial with IBM.
It also helped that at the time there were several "disk operating systems" for various platforms, which is why the MS- part on MS-DOS was important.
That really explains a lot, and why MS jumped to Windows NT, they knew that DOS would always be Dirty.