Have you ever seen "AIN'T"? Where? Maybe in a lyric song? What does it mean?
Well, according to some linguists, AIN'T comes from the contraction of AM NOT and it seems to have appeared in 1706. One century later, in Cockney dialect in London, AIN'T began to be used as a generic contraction for ARE NOT or IS NOT. Then, it was also used as a contraction of has not and have not. Even more, in some dialects, does not or do not can be replaced by AIN'T.
As you can imagine or see, AIN'T is a colloquialism. So those who use this are regarded as uneducated, informal or improper people. However, I can give some famous examples: "Ain't No Mountain High Enough" or "Ain't No Sunshine When She's Gone".
Therefore, I think it's interesting to know the origin and uses of this word, we just want to learn English, no matter if it's not always the Standard proper English ;)