Source : Free On-Line Dictionary of Computing

humma
     
         A filler word used on various "chat" and "talk"
        programs when you had nothing to say but felt that it was
        important to say something.
     
        The word apparently originated (at least with this definition)
        on the MECC Timeshare System (MTS, a now-defunct educational
        {time-sharing} system running in Minnesota during the 1970s
        and the early 1980s) but was later sighted on early Unix
        systems.
     
        [{Jargon File}]
     
        (1999-02-27)