I'll be a monkey's uncle

(Well) I'll be a monkey's uncle!

An expression of surprise or amazement. Well, I'll be a monkey's uncle—Shane actually managed to get his movie made.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

I'll be a monkey's uncle

used to express great surprise. informal
See also: uncle
Farlex Partner Idioms Dictionary © Farlex 2017

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
See also:
References in periodicals archive ?
And if further office visits alter that diagnosis, well, I'll be a monkey's uncle.