1
general::
Also, what's the big idea; the very idea. What do you think you are doing? What foolishness do you have in mind? For example, What's the idea of taking the car without permission? or You've invited yourself along? What's the big idea? or Take a two-year-old up Mount Washington? The very idea! These phrases, all implying the speaker's disapproval, use idea in the sense of "what one has in mind." The first two date from about 1900; the third is heard more in Britain than America.
American Heritage Idioms
2
general::
Phrase(s): What’s the (big) idea?
Inf. Why did you do that? (Usually said in anger.) • Please don’t do that! What’s the idea? • Why did you shove me? What’s the big idea?
McGrawhill's American Idioms And Phrasal Verbs