"Ain´t" is slang, and could mean any of the following: "am not", "are not", "is not" "has not" "have not". "You ain´t seen nothing yet" means "You haven´t seen anything yet". "I ain´t coming home" means "I am not coming home" etc

Go ask Google or Urban Dict.

Personally, I find it perfectly fine as a first person singular contraction of the conjugation "to be" The real one: Aren't I cool? But you're saying, "Are I not cool?" which doesn't really make sense. I suggest ain't as a better alternative. But nobody ever listens to me. :P

is not ain't ain't a word! :P

It is typically used by lower class, uneducated people. In other words, don't use use it!