The term “monkey’s uncle,” in the idiom “well, I’ll be a monkey’s uncle!” is used to express complete surprise, amazement or disbelief. It can also be used to acknowledge the impossibility of a situation, in the same way that “pigs might fly” is used.
The term is assumed to be a reference to Darwin’s Origin of Species of 1859, in which he argued the close evolutionary relationship between humans, apes and monkeys.
Etymologist Michael Quinion notes that the phrase “monkey’s uncle” occurs in a parody of Longfellow’s The Song of Hiawatha which was reprinted in James Parton’s The Humorous Poetry of the English Language, published in 1881, some 22 years after Darwin’s book was published.
In spite of that, some reference books suggest it dates from the 1920s. One such reference dates to 1925, the year of the widely publicized Scopes Trial in the US, where the term appeared. The earliest example quoted in the Oxford English Dictionary is: “If that’s a joke I’m a monkey’s uncle,” from an Ohio newspaper on 8 February 1925.
It was originally a sarcastic remark made by creationists. The notion “that [people] were descended from apes was considered blasphemous…by Darwin’s contemporaries,” and it was for this reason that the sarcastic phrase came into use.
A couple of films titled “Monkey’s Uncle” have been made. Also, on their 2003 album Reel to Real, The Selecter included a song titled “Monkey’s Uncle,” criticizing religious dogma that contradicts scientific evidence.
Have you noticed that “monkey’s uncle” is almost a rhyming phrase? It’s the repetition of the “unk” sound. And we do love rhyming phrases. Which means that being a “monkey’s aunt” or a “monkey’s cousin” is never going to hit the big time.