1-a response to subordinate type people ( ie new hires youngest children, people sent to persude you by their cuteness but are really manipulative and you see right through them) basically meaning don't try because you don't have a chance .
2- a response to people who are unfortunate enough to be the second third or fourth person to ask you for the same thing,. in other words- they have no chance!
Taken from the Zack character on "saved by the bell" and the Zakachary character on "officer and a Gentleman.
Newbie: " Can we get off early today?"
Boss: "forget it Zack!"
Youngest child: "I cant sleep. Can I have a glass of water?"
Parent:" Forget it Zack! You and your brothers are NOT staying up! Nice try."
Girl:"He wanted to borrow some money. I told him " forget it Zack, your the third person to ask this week!"