1. a combination of the words are not.
2. a combination of the words am not.

Short for "am not" and for nothing else – not even "is not", for "it ain't" would just be "it am not". The only correct use is "I ain't" which is a contraction of "I am not". So, don't say e.g. "you ain't" which would only mean "you am not".

Originally a non-slang word, ain't began as a contraction for "am not", as in "I ain't stupid." When people began to use it improperly as in "He ain't", which should have been "He isn't", people came to consider it improper use of language. Today, it's known to be used predominantly in the south and by uneducated people.

Contraction of am not and slang contraction for anything else.

A word that kids used to get yelled at for saying because it wasn't a word. Though, That is a no longer a case. Ain't is in the dictionary so you can go find your 2nd grade teacher and tell her to piss off.

Is not.

A word used as a short form of am not, is not, or anywhere else you have heard it.