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

real operating system
 n.

    The sort the speaker is used to. People from the BSDophilic academic
    community are likely to issue comments like ?System V? Why don't you use a
    real operating system??, people from the commercial/industrial Unix sector
    are known to complain ?BSD? Why don't you use a real operating system??,
    and people from IBM object ?Unix? Why don't you use a real operating
    system?? Only MS-DOS is universally considered unreal. See holy wars, 
    religious issues, proprietary, Get a real computer!


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

real operating system

    The sort the speaker is used to.
   People from the BSDophilic academic community are likely to
   issue comments like "System V?  Why don't you use a *real*
   operating system?", people from the commercial/industrial
   Unix sector are known to complain "BSD?  Why don't you use a
   *real* operating system?", and people from IBM object "Unix?
   Why don't you use a *real* operating system?"

   See holy wars, religious issues, proprietary, Get a
   real computer!.

   [Jargon File]

   (1997-03-12)