Usage of "ain't"?
Ain't is used as a regular negated form of be or have, and supposedly sometimes do:
I ain't no tractor. = I am not a tractor.
I ain't got no tractor. = I haven't got any tractor.
It's also used like there isn't, by common omission of there from there ain't.
- Ain't no tractor here. = There isn't any tractor here.
And in case you hadn't guessed, dialects that use ain't stereotypically use negative concord as well.
I was taught that "ain't" is a contraction of "am not." Its wide misuse has caused it to be considered slang.