As in, "Well, I'll be a monkey's uncle!" This phrase means, "That's amazing!" or "I never would have beleived that!" A person says this of themselves only, and it is always meant to imply that the speaker is amazed or surprized by what they have just heard. It originally comes from people being amazed at Darwin's Theory of Evolution, which set forth that human beings and monkeys evolved from the same common ancestor.
Learn more English idioms at http://www.usingenglish.com/reference/idioms/
