Term used to signify that you are really focused on your work, often used as an excuse to shoo away inquiring co-workers. Literally means you have your head down permanently affixed to your computer screen.
A phrase made popular by a pro gamer named FPS Doug on the internet series Pure Pwnage. FPS Doug says "Boom, headshot" everytime he shoots an opponent in the head in Counterstrike Source or any first person shooter type game.