What is a monkey's uncle?

The Monkey's Uncle is a slang expression for "really something."

The slang term "I'll be a monkey's uncle" means that one is greatly surprised by a fact or revelation. It originated around 1925 when the Scopes Monkey Trial (on evolution) was under way.

The Disney movie "The Monkey's Uncle" (1965) was about an inventor and his human-powered aircraft. The title comes from a unique legal arrangement whereby an inventor (played by Tommy Kirk) becomes the "uncle" of a chimp being raised among humans.