Rapid stop and go typing that one does when instant messaging. Can be identified by the tell tale sounds of:

tappity-tappity-tappity-tappity-tappity-tap-tap-tap-CLICK
*pause*
tappity-tappity-tappity-tappity-tap-tap-tap-CLICK
*pause*
tappity-tappity-tap-tap-tap-tap-tap-tap-tap-tap-CLICK
and so on...

Named because it sounds like and has a similar rhythm to firing an automatic weapon. Usually not that annoying, but can be incredibly irritating when done by that lazy co-worker always slacking off at work or that college roommate that IMs until 5am every night.
Also known simply as "machine gunning".