Source : Free On-Line Dictionary of Computing

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}]