Dictionaries say ain't is a contracted form of am not, is not, are not, has not, or have not.
The Cambridge Dictionary gives the following example sentences as well.
He ain't going.
"Can I have a cigarette?" "I ain't got none left."
I ain't done nothing wrong.
Lend me a fiver - I ain't got no money right now.
Example sentences 2-4 have negative words none, nothing, no respectively in them, even though ain't is already there to make them negative. Why this double negative ?
Isn't the sentence 1 negative without another negative word in it?