Green’s Dictionary of Slang

monkey’s uncle n.

In exclamations

I’ll be a monkey’s uncle! (also I’m a clam’s uncle! I’m a monkey’s aunt! I’m a monkey’s uncle!)

a general expression of surprise.

1923		Chronicle-Telegram (Elyria, OH) 8 Feb. ?/3–4: ‘The nuptial idea is a joke,’ she told Lawrence. But Lawrence can’t see it. ‘If that’s a joke I’m a monkey’s uncle.’.
1926		Maines & Grant Wise-crack Dict. 5/2: Be a monkey’s uncle – Be surprised.
1939		R. Chandler ‘Pearls Are a Nuisance’ in Spanish Blood (1946) 121: Why, I’ll be a monkey’s uncle.
1941		J. Archibald ‘Alibi Bye’ in Popular Detective June 🌐 ‘Well, I’m a monkey’s aunt,’ Willie said.
1943		R.L. Bellem ‘Phoney Shakedown’ Dan Turner - Hollywood Detective Feb. 🌐 I glued the startled glance on her. ‘Well, I’ll be a monkey’s uncle!’.
1949		(con. 1943–5) A. Murphy To Hell and Back (1950) 98: If it works [...] I’m a blue-tailed monkey’s uncle.
1960		P. Rabe Murder Me for Nickels (2004) 115: Well, I’ll be a monkey’s uncle.
1974		(con. 1930s) I. Agnew Loner 94: Well, I’ll be a monkey’s uncle! Who’d have thought he had it in him?
1974		R. Shiarella Your Sparkle Cavalcade of Death 121: Mister Big Shot. If he ain’t a cow fag, then I’m a clam’s uncle.
1989		H. Rawson Dict. of Invective (1991) 254: The surprised exclamation, ‘I’ll be a monkey’s uncle!’.
1994		(con. 1960s) G. Byrne Pictures in my Head 40: She told us she was a monkey’s uncle and the sun of a gun and that her timbers were shivering.
2000		C. Cook Robbers (2001) 90: Well, I’ll be a monkey’s uncle, the man said.
2021		P. Papathanasiou Stoning 38: ‘Well, I’ll be a monkey’s...’.