SABABWL is an acronym for a quote by Martin O'Donnell back when the original Halo was about to be released. It stands for "shiny and bumpy and bursting with love!" This is a comical outlook on the Grunts' appearance, but not so much their behavior. Although Grunts do have a shiny and bumpy exoskeleton with a peculiar appearance, the "bursting with love" bit may not be so accurate, as most all of the Grunts in the Halo Trilogy try to kill the player, although the comment may be referring to the close relationship between Grunts.
It might also just be a simple joke or exaggeration referring to their position as the comic relief and general popularity with the fans.