Green’s Dictionary of Slang

a dismissive phr. indicating one’s lack of concern.

1844		‘Jonathan Slick’ High Life in N.Y. 68: I didn’t exactly like the feel of it, but ‘Who cares’, sez I to myself.
1989		M. Amis London Fields 275: ‘Has she any qualifications?’ ‘I didn’t ask.’ ‘Has she any references?’ ‘Who cares?’.
1993		R. Firestone Swing, Swing, Swing 399: We weren’t working for him every week, you know. Who cared?
2000		Guardian G2 19 Jan. 6: Like, who cares?