A catch phrase popularized by the video "My New Haircut." Said when someone is on a roll or in a state of extreme focus and does not wish to be interrupted by someone that he/she deems unimportant and irrelevant.
From "My New Haircut":
Guy at desk: "Sir, you have to sign in."
Guido: "Not now chief, I'm in the fucking zone."
Kindergarten Teacher: "Who wants to read the next couple pages of 'The Cat in the Hat?'. Let's see...Jimmy, how about you?"
Jimmy: "Not now chief, I'm in the fucking zone."
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