Definition & Meaning of "[be] a monkey's uncle"

What is the origin of the idiom "be a monkey's uncle" and when to use it?

The origin of the idiom "be a monkey's uncle" can be traced back to the famous Scopes Monkey Trial that took place in 1925 in the United States. During the trial, a lawyer sarcastically asked the judge if he wanted to "make a monkey out of me" by allowing the opposing lawyer to present evidence. The judge responded with the phrase "I'm not trying to make a monkey out of anyone. I'm trying to establish whether or not you are related to the monkey." The phrase "be a monkey's uncle" since then, gained popularity as a humorous way to express skepticism or to mock someone's claim.