There are several good online references that explain the root and meaning of the word
ain't. Here are some:
Ain't is a slang contraction originally used for "am not", but also used for "is not", "are not", "has not", or "have not" in the common vernacular. In some dialects it is also used as a contraction of "do not", "does not", and "did not" (i.e. I ain't know that). The word is a perennial issue in English usage. It is a word that is widely used by many people, but its use is commonly considered to be improper.
http://en.wikipedia.org/wiki/Ain't
1 : am not : are not : is not
2 : have not : has not
3 : do not : does not : did not —used in some varieties of Black English
http://www.merriam-webster.com/dictionary/ain't
1. Contraction of am not.
2. Used also as a contraction for are not, is not, has not, and have not.
http://www.thefreedictionary.com/ain't