then I'm a monkey's uncle

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then I'm a monkey's uncle

An expression of utter disbelief or incredulity. Used after a conditional phrase. A: "Tom claims he's descended from Napoleon Bonaparte." B: "Ha, yeah right. If that's true, then I'm a monkey's uncle!"
See also: uncle
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

monkey's uncle, I'll be/am a

An expression of surprise or disbelief. Why that particular animal was chosen is no longer known, and indeed the cliché, which dates from the early 1900s, is heard less often today. Eric V. Lustbader used it in White Ninja (1990): “If this doesn’t turn out to be a suicide, I am a monkey’s uncle.”
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
References in periodicals archive ?
If that's giving power back to the voter, I'm a monkey's uncle.
But if the BNP is in with a shot of winning elections in Brum, then I'm a monkey's uncle.