big deal

An important matter; also, so what, who cares. The differentia- tion between these two phrases depends on the speaker’s tone. An example of the first sense might be, “A first helicopter ride for a five-year-old is a big deal.” The  second  sense  appears  in,  “So  she  got  her  first  choice  of  col- leges—well, big  deal!” This  sense  can  also  be  conveyed  by  no  big  deal. All three usages date from about 1940.