Green’s Dictionary of Slang

(US) a euph. for god-damn adj.

c.1935		‘She Will Be Coming Around the Mountain’ [comic strip] in B. Adelman Tijuana Bibles (1997) 51: Take that you dad-burn idjit varmit.
1940		F. Hunt Long Trail from Texas 39: You dad-bern [sic], Arkansas thowoffs!
1957		‘Weldon Hill’ Onionhead (1958) 189: ‘[W]e might throw it [i.e. a scavenging buzzard] one of them dadburn cooks’.
1972		C.E. Palmer Wooing of Beppo Tate 79: I think he’s the one to make a move and kiss the dadburn girl.