Green’s Dictionary of Slang

(orig. US) damned if I know!

1871		Eaton Wkly Democrat 2 Mar. 2/2: ‘What are you going to do after you graduate?’ said a gentleman to a Williams College student [...] ‘Damfino,’ replied the youth.
1883		G.W. Peck Peck’s Bad Boy and His Pa (1887) 25: Ma said [...] where is the huzzy, and a lot of things I couldn’t hear, and Pa said ‘damfino’.
1886		Sporting Times 8 May 5/5: ‘Where’s your hat, dear?’ ‘Damfino!’ / Then everything blank.
1893		St Paul Dly Globe (MN) 29 May 4/8: The young man [...] blurted out: ‘Damfino’.
1895		Truth (Sydney) 14 Apr. 3/2: ‘Damfino, “Outsider,” old man,’ he said. ‘What’s the odds?’.
1900		Sporting Times 6 Jan. 2/1: Damfi—I should say, that is what has cost me many a sleepless night to determine.
1904		‘Hugh McHugh’ I’m from Missouri 29: Question – What is the heinous crime referred to? Answer – Damfino.
1905		Sporting Times 1 Apr. 3/2: What the delaval that tarantula of a landlord of mine will say when he’s given the whisper on quarter day, damfino.
1911		S.F. Call 5 Mar. 8/2: Bill’s characteristic ‘damfino, I never eat here,’ was the reply.
1925		(con. 1899) H.P. Bailey Shanghaied Out of Frisco 53: ‘Damifino!’ he returned.
1928		(con. 1918) L. Nason Top Kick 14: ‘Damfino,’ said Nell, ‘it’s a Boche sign.’.
1987		(con. 1967) Bunch & Cole Reckoning for Kings (1989) 7: Damfino, but good guess.