|slang an exclamation of scorn, derision, etc, used esp to belittle a claim or offer|
A matter of great interest or importance, as in Performing in Symphony Hall is a big deal for everyone in the chorus. [c. 1940] Also see under make a federal case out of.
So what? Who cares? For example, So you got the job after allwell, big deal! This use of the phrase as an ironic interjection dates from approximately the same time.