An idiomatic expression that implies surprise, shock and/or disbelief.

"I'll be a monkey's uncle" is often preceded by "Well", as in "Aunt Edna is really a man!?! Well I'll be a monkey's uncle", and thus is abbreviated as WIBAMU in Netspeak.

I have tried to track down the origin of the phrase, but I have had no luck. It may have something to do with evolution...

I'll be a monkey's uncle was originally used as a sarcastic remark, used to express disbelief. When Darwin's Descent of Man,was published in 1874 the idea that man could be descended from monkeys was ridiculed and considered outrageous. The phrase was used to show scorn for Charles Darwin's theories.