Green’s Dictionary of Slang

to be of no consequence, to not matter at all.

1894		A. Morrison Tales of Mean Streets (1983) 117: You ’ave to shed blood in a revolution, an’ a few odd lives more or less don’t matter — not a single damn.
1922		‘Sapper’ Black Gang 412: It doesn’t matter a damn what happens to us.
1929		(con. 1916) F. Manning Her Privates We (1986) 32: Have it your own way, then; it didn’t matter a damn anyway.
1941		K. Tennant Battlers 203: Life just flowed along, and it didn’t matter a damn what you wanted.
1955		D. Niland Shiralee 180: I suppose it doesn’t matter a damn, anyway.
1975		Sun. Times Mag. 12 Oct. 36: By that time money didn’t matter a damn.
1982		G.V. Higgins Patriot Game (1985) 49: It doesn’t matter a damn if he did something before he signed your register.
1999		I. Rankin Dead Souls 4: It wouldn’t matter a damn.