{informal} What is the purpose; what do you have in mind; why did you do that; what are you doing; how dare you. - Often used to question someone or something that is not welcome. * /The Smith family painted their house red, white, and blue. What's the big idea?/ * /What's the idea of coming in here after I told you not to?/ * /I heard you are spreading false rumors about me, what's the big idea?/
See: WHAT'S THE BIG IDEA.