A popular BackroNym for "TOS", the OperatingSystem which ran on the AtariSt series of computers. (In reality, TOS didn't stand for much of anything). JackTramiel?, who founded Commodore but was subsequently ousted, took over as head of Atari after Warner Brothers decided they didn't want to be in the computer business. Many hoped he would pull Atari out of its nose-dive; didn't happen.
Largely a clone of MacOs; but with a few subtle changes to avoid Apple's lawyers (who sued Microsoft instead), and the 8.3 filename limit (a bit of CP/M legacy that MS would happily preserve well into the next decade) intact.
More correctly, TOS is the collection of parts that made up the operating system, consisting primarily of GEMDOS which was vaguely a clone of MS-DOS, resulting in the Atari ST using FAT in all its 8.3 glory as its native filesystem, and a GEM GUI licensed from DRI. DRI did in fact get sued by Apple over GEM, resulting in the crippled GEM/2 on the PC. Atari's GEM was fully forked from DRI at version 1.0, and was never sued by Apple, thus the ST's GEM remained uncrippled for its entire lifetime.