Of course, strictly speaking, "Ain't nothin'" means that there is something...
Actually, no, not really. "Ain't nothin'" belongs to African-American English, which is not just a "wrong" dialect of English. It actually formed over several centuries and has words and especially grammatical forms from several languages (based on countries where Africans were slaves). In this dialect, double negative is the standard, just as double negative is correct in French (I believe that this particular grammar came from Haiti, if I remember my linguistics classes right).
At least, that's from a linguistic point of view.
