Green’s Dictionary of Slang

(US) a euph. for damned adv.

1834		‘Jack Downing’ Andrew Jackson 24: But the gineral [...] wrote a consarnt cute letter to Jefferson.
1844		‘Jonathan Slick’ High Life in N.Y. I 165: Jase trod on my toe with his consarned hard boot. [Ibid.] 178: She was a slick leetle critter, with the consarnedest soft eyes.
1882		E.S. Ellis Huge Hunter in Beadle’s Half Dime Library XI:271 5/1: By the jumpin’ jingo! if we was consarnedly mean enough to do it, there ain’t no need of it.
1905		DN III 6: I’m consarned sorry for it.