The Jargon File (version 4.4.7, 29 Dec 2003):

mess-dos
 /mes?dos/, n.

    [semi-obsolescent now that DOS is] Derisory term for MS-DOS. Often followed
    by the ritual banishing ?Just say No!? See MS-DOS. Most hackers (even
    many MS-DOS hackers) loathed MS-DOS for its single-tasking nature, its
    limits on application size, its nasty primitive interface, and its ties to
    IBMness and Microsoftness (see fear and loathing). Also mess-loss,
    messy-dos, mess-dog, mess-dross, mush-dos, and various combinations
    thereof. In Ireland and the U.K. it is even sometimes called ?Domestos?
    after a brand of toilet cleanser.


The Free On-line Dictionary of Computing (30 December 2018):

mess-dos

   /mes-dos/ (Or MS-DOG, Messy-DOS, mess-dross, mess-loss,
   mush-dos) Derisory term for MS-DOS.  Often followed by the
   ritual banishing "Just say No!"

   Most hackers (even many MS-DOS hackers) loathe MS-DOS for
   its single-tasking nature, its limits on application size, its
   nasty primitive interface, and its ties to IBMness (see
   fear and loathing).

   In Ireland and the UK it is sometimes called "Domestos" after
   a brand of toilet cleanser.

   [Jargon File]

   (1994-11-16)