Source : Free On-Line Dictionary of Computing

BUAG
     
        [alt.fan.warlord] Big Ugly ASCII Graphic.  Pejorative term for
        ugly {ASCII ART}, especially as found in {sig block}s.  For
        some reason, mutations of the head of Bart Simpson are
        particularly common in the least imaginative {sig block}s.
     
        See {warlording}.