The Free On-line Dictionary of Computing (19 January 2023):

sun-stools

   Unflattering hackerism for SunTools, a pre-X windowing
   environment notorious in its day for size, slowness, and
   misfeatures.  X, however, is larger and slower; see
   second-system effect.


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

sun-stools
 n.

    Unflattering hackerism for SunTools, a pre-X windowing environment
    notorious in its day for size, slowness, and misfeatures. X, however, is
    larger and (some claim) slower; see second-system effect.