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!