When you are doing something at work that is not work-related that requires your full attention, such as playing a video game, and a co-worker suddenly pops in forcing you to make it look as if you are busy and hide your game (such as with alt-tab). After the co-worker leaves, you typically return to your video game to find your character dead.
Joe: Hey man what are you doing? Are you gonna help us capture the flag?
Bob: Sorry I got coworked! Had to alt-tab!