Machine Gun Typing 

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".

Coworker 1: "Can you assist us with this project? We need everybody's help, because there's a ton of work."

Coworker 2: "I'm really busy right now. Kind of overwhelmed."

Coworker 3: "Bullshit! All you're doing is IMing your friends. It's pretty damn obvious by your constant machine gunning. Now do your frikkin' job."

I really needed to crash because I had the spins from partying too hard, but I was kept up all night by my herb of a roommate machine gun typing to girls he'll probably never meet in real life.

Machine Gun Typing by Toonces the Driving Kat May 16, 2010