I’m a nigger, phr. — Green’s Dictionary of Slang

I’m a nigger phr.

also I’m a coyote, ...horned sinner, ...monkey, ...monkey’s uncle, ...ring-tailed baboon

a phr. used to refute a suggestion or hypothesis, and as an excl. of astonishment.

1832		J.K. Paulding Westward Ho! II 100: What ignorant Turks these Indians are [...] I’m a nigger if I think this copper-washed man is a right clean, full-blooded feller-cretur.
1897		A.H. Lewis Wolfville 134: ‘Well, I’m a coyote!’ says Monte, a heap disgusted, ‘wherever is the female?’.
1926		Z. Grey Fighting Caravans (1992) 104: I’m a horned sinner if some gang ain’t robbin’ the wagon!
1930		(con. 1910s) L. Nason A Corporal Once 41: If he wasn’t doin’ sixty miles a hour, I’m a nigger.
1950		A.B. Guthrie Way West 308: Be more broke wagon tongues today, or I’m a nigger.
1967		J. Schaefer Mavericks (1968) 92: Iffen that ain’t a hoss [...] I’m a ring-tailed baboon.
1975		T. Berger Sneaky People (1980) 143: ‘I never even done it before,’ said Laverne. ‘No lie?’ said Ken. ‘Well, I’m a monkey’s uncle.’.
2001		S. King Dreamcatcher 92: She’s been drinkin somethin, by Christ, and if it ain’t antifreeze, I’m a monkey.