The term monkey’s uncle, most notably seen in the idiom “I’ll be a monkey’s uncle”, is used to express complete surprise, amazement or disbelief (sort of like “when pigs fly”).
In the 19th century, Charles Darwin published two books, ‘On the Origin of Species’ (1859) and ‘Decent of Man’ (1871). In both of these books, Darwin shared his theory of evolution: that man evolved from apes/monkeys.
Many people did not agree with Darwin’s theories and the phrase “I’ll be a monkey’s uncle” began to be used sarcastically by non-believers.
“If what you told me is true then I’ll be a monkey’s uncle!”