The story
Uncertain; the term monkey’s uncle appears in 19th-century works and may allude to early ideas about what is now called the theory of evolution. The term may then have gained more currency after the widely publicized 1925 Scopes Trial in Tennessee, United States, in which a high-school teacher was found guilty of having violated a law prohibiting the teaching of human evolution in a state-funded school.
Etymology from Wiktionary, CC BY-SA 4.0.