MS-DOS, the most widely used operating system of the 1980s, is short for Microsoft Disk Operating System.
The name was influenced by its predecessor 86-DOS (named after the Intel 8086-based computer kit it was made for) and originally called QDOS (Quick and Dirty Operating System). Bill Gates and Paul Allen bought the rights to the system from Seattle Computer Products for $50,000 in July 1981. Microsoft would modify it for PCs and release it under the name MS-DOS and PC-DOS (a version specifically for the IBM PC) in August of the same year.
Sources: 86-DOS | MS-DOS Github repo