What does "Your saying so don't make it so" in the following mean and is it grammatically valid?
“You’re a coward and a pup. I’ll tell my big brother on you, and he can thrash you with his little finger, and I’ll make him do it, too.”
“What do I care for your big brother? I’ve got a brother that’s bigger than he is—and what’s more, he can throw him over that fence, too.” [Both brothers were imaginary.]
“That’s a lie.”
“Your saying so don’t make it so.”
It's from The Adventures of Tom Sawyer, chapter 1.
If it was "You're saying so, don't make it then/so.", I would say it means "You yourself are lying. So don't call me a lier.". But this structure is a little strange to me.
Could it be a proverb, the whole sentence?
Or maybe "Your saying" here means "What you say" and then he tells the other person not to do so (you yourself do not lie).
Or could it be some reductions/changes because of the dialect?