GOT IT!
Ain’t can mean “am not” or “is not”, therefore it is Am/Is Not, therefore it is AI not, or “ain’t”
Because, lets face it, you really only use it if you’re not sure if the sentence calls for an “am” or an “is”.
“Ain’t can also mean “are not”, but “aren’t” isn’t much harder to say than “ain’t”, soo…
@DominicX whilst willn’t is a lot of fun do say, it just doesn’t quite have the same effect as won’t.