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!

Fig. I am amazed! A: I just won $500,000 in the lottery! B: Well, I'll be a monkey's uncle!
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
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