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...