Similar to the expression “in your dreams” which is a sarcastic way of responding to - or interrupting – someone who is busy telling you about their master plan and all the unrealistically fantastic things they think will happen to them. The difference is in this version you are making it clear they should get over themselves
Steve: I’m going to win this race and humiliate you in the process
Harry: Quit dreaming! That’ll never happen.
Harry: Quit dreaming! That’ll never happen.
by The Captive Spirit September 13, 2010