Green’s Dictionary of Slang

a widely used term of disinterest or defiance, a rejoinder to the previous speaker’s announcement, revelation etc.

1932		R. Fisher Conjure-Man Dies 138: ‘So what?’ inquired Detective Dart.
1946		B. Cerf Anything For a Laugh 168: ‘So what?’ sneered the American bluejacket.
1959		W. Hall Long and the Short and the Tall Act I: evans: He’s right, Bammo! I heard it myself. bamforth: Ah, so what.
1966		P. Willmott Adolescent Boys of East London (1969) 64: She’s nasty. If I say something to her, she’ll say ‘So what’.
1976		N. Thornburg Cutter and Bone (2001) 9: So the old stomach acts up again — so you get wound up — so what?
1981		S. Berkoff Decadence in Decadence and Other Plays (1985) 35: So what he’s off with his high-class dame / so what, for them life’s just a game.
1995		M. Simpson ‘Prufrock Scoused’ Catching Up with Hist. 23: So bleedin wot?
2000		P. Cornwell Last Precinct 369: Well, so fucking what?