An Acronym for:

Denial Of Service
Usually written "DoS" - see DenialOfService

OR

Disk Operating System. Usually named after the operating systems of 8bit micros, who's operating system didn't include memory management, multiuser or multiprocessing and only a thin abstraction if any over any hardware, so they only part of the Operating System that they actually implemented was the filesystem, hence, DOS.

MS-DOS (Microsoft DOS) was originally called QDOS, and was written in 2 weeks. QDOS stands for "Quick and Dirty Operating System". Microsoft promised IBM that they would cheaply license them an OS - they bought the rights to QDOS and started hacking it up from there...