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2.
The alternative to the overused "ohp nevermind" which has more class and wit. Used when someone says something that the speaker wishes to reject or shoot down.
Person A: "Did you just see that badass backflip I just pulled off? It was amazing!."

Person B: "Dream on Dennis Quaid"
by Mr. Councilman April 07, 2011
 
1.
Used to shoot down someone's thoughts, actions, or feelings in a witty and formal way.

It is a reference to the move, Dreamscape, where Dennis Quaid plays the main role. He enters people's dreams and helps them conquer their fears.
Person A: "I've been wanting to ask you this for a long time...will you marry me?"

Person B: "Dream on Dennis Quaid"
by Mr. Councilman April 07, 2011