According to co-founder Paul Allen, who was still working at Microsoft in 1981, the D in DOS actually stood for DIRTY, as in Down and Dirty OS - or - Quick and Dirty OS.
Since then, the consensus has become Disk or Digital OS.
The original creator of DOS refused to sell for a fixed price, asking for royalties instead.
IBM declined that offer.
Bill Gates discovered a computer parts manufacturer that was using a variant of DOS to test their Intel chip powered components coming off the production line.
According to legend, Gates paid $50,000 for full rights to the chip tester OS.
The rest of the story is business history!
If we're going to get accurate, the original operating system named "DOS" was "DOS/360" for the IBM/360 computers, and it wasn't in the 80's, it wasn't in the 70's, it was in the mid-60's while guys like Gates were still in middle school.