1851, American English, a "fantastic perversion of god-damned" [Weekley]. But Mencken favors the theory that it is "a blend form of dog on it; in fact it is still often used with it following. It is thus a brother to the old English phrase, 'a pox upon it,' but is considerably more decorous."
(also doggoned or daggoned or dagnabbed) Wretched; nasty; silly: This doggone thing's busted (1860+)
: They left here doggone fast (1871+)
[probably a euphemism for God damn]