What does the idiom well be a uncle mean?

The idiom "well be a uncle" doesn't actually exist. There is no common idiom or phrase that uses those words.

It's possible you're thinking of a similar expression, or perhaps you've heard a variation that's not widely recognized.

To help me understand what you meant, could you tell me:

* Where did you hear this expression? Was it in a book, movie, song, or spoken conversation?

* What was the context? What were the speakers talking about?

Knowing more about the situation might help me figure out what you're looking for!