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

USG Unix

   operating system /U-S-G yoo'niks/ Refers to AT&T Unix
   commercial versions after Version 7, especially System III
   and System V releases 1, 2, and 3.  So called because during
   most of the lifespan of those versions AT&T's support crew was
   called the "Unix Support Group".

   Compare BSD.

   [Jargon File]

   (1997-02-20)