When playing Call of Duty Zombies with friends, "zombie sitting" is the act of making the last zombie of the round into a crawler, and having one person maintain the zombie's agro, while the other friends use the restroom or grab munchies. This is required in games that last a few hours, as there is no pause button, in online play.
Yo dude, can you zombie sit for me. I've gotta pee.
Dude, I need a zombie sitter, I'm hungry.