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

TOPS-10
 /tops?ten/, n.

    DEC's proprietary OS for the fabled PDP-10 machines, long a favorite of
    hackers but now long extinct. A fountain of hacker folklore; see Appendix
    A. See also ITS, TOPS-20, TWENEX, VMS, operating system. TOPS-10
    was sometimes called BOTS-10 (from ?bottoms-ten?) as a comment on the
    inappropriateness of describing it as the top of anything.


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

TOPS-10

   /tops-ten/ DEC's proprietary OS for the fabled PDP-10
   machines, long a favourite of hackers but now effectively
   extinct.  A fountain of hacker folklore.  See also ITS,
   TOPS-20, TWENEX, VMS, operating system.  TOPS-10 was
   sometimes called BOTS-10 (from "bottoms-ten") as a comment on
   the inappropriateness of describing it as the top of anything.

   [Jargon File]