OS/2

OS/2 /O S too/ n.  The anointed successor to MS-DOS for
   Intel 286- and 386-based micros; proof that IBM/Microsoft couldn't
   get it right the second time, either.  Often called `Half-an-OS'.
   Mentioning it is usually good for a cheap laugh among hackers ---
   the design was so {baroque}, and the implementation of 1.x so
   bad, that 3 years after introduction you could still count the
   major {app}s shipping for it on the fingers of two hands -- in
   unary.  The 2.x versions are said to have improved somewhat, and
   informed hackers now rate them superior to Microsoft Windows (an
   endorsement which, however, could easily be construed as damning
   with faint praise).  See {monstrosity}, {cretinous},
   {second-system effect}.