You could be forgiven for thinking that a monkey's uncle was the brother of one of a monkey's parents. Technically, you wouldn't be wrong, but you would probably be laughed at. It is in fact more widely known as an exclamation of surprise when you find out that something is true that you otherwise believed not to be so. For example, you might say, "If that cat's a fish, then I'll be a monkey's uncle!" This is not to be confused with the other popular phrase "Bob's Your Uncle". And the two should definitely not be combined to produce the nasty rumour, "Bob Monkhouse is an Uncle".

In any of the above phrases, the likelihood would be that you are either wrong or lying, and both should be avoided if at all possible.