What does DOS stand for?

Question: DOS - Disk Operating System. But what did DOS originally stand for?

Answer: "Dirty" Operating System

Background History from inventors.about.com and wikipedia.org

Click, double-click, drag-and-drop. Although almost everything is now GUI, we still utilize DOS on occasion. Even after 25 years, DOS is still available because of its speed and efficiency in certain cases. However, what most of us don’t know is that DOS did not stand for “Disk Operating System” originally. In order to explain what DOS really stands for, let’s start with a short background of how it came about.

The need for DOS arose in the 1980s, when IBM was developing a computer (IBM Personal Computer) but did not have an Operating System to use. Since CP/M (Control Program for Microcomputers) from Digital Research was a popular OS used during this time, IBM had considered using CP/M as their OS. Because Gary Kildall, the creator of CP/M, did not handle business negotiations himself, IBM executives worked with his former wife, Dorothy McEwen, and Kildall’s attorney. However, with the non-disclosure agreement that remained unsigned, the refusal to modify CP/M, and higher royalty requested than IBM’s proposal, Digital Research lost the opportunity to have their technology used by IBM.

Since IBM couldn’t use CP/M as their OS solution, they turned to Microsoft. Although Microsoft was a CP/M subcontractor, their contract would not permit them to subcontract CP/M for IBM. Therefore, Microsoft purchased a nonexclusive license for QDOS, or Quick and Dirty Operating System, from Seattle Computer Products, and hired Tim Paterson to do the porting.

The license Microsoft used to provide IBM with QDOS (or PC-DOS, as IBM renamed it) allowed themselves to sell this “DOS” to other companies as well, which is when the MS-DOS was first released. Since Microsoft’s DOS was created from QDOS, you could say that the “D” in DOS originally stood for “Dirty”.