"What's the big idea" is a phrase
used whenever a person has a stupid,
impossible, or pointless idea and
someone needs to tell them off without being too rude.
Jason: "Dude, let's go smoke
weed in public!"
Mike: "Oh, really? What's the big idea? Get arrested?"
Jason: "Uh, no...it's to have fun."
Mike: "In jail? Are you barred again?"