What's the idea?

What's the idea?

A question that expresses frustration or annoyance about what is happening and asks (at least rhetorically) for the reason or motivation behind it. Now just what's the idea, here? I saw you dancing with some other guy out there! Hey, what's the idea? Why are you wearing my favorite dress? What's the idea, shoving me like that?

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?

what's the idea

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.
See also: idea