The Collaborative International Dictionary
Ain't \Ain't\ A contraction for are not and am not; also used for is not. [Colloq. or illiterate speech]. See An't.
Douglas Harper's Etymology Dictionary
1706, originally a contraction of am not, and in proper use with that sense until it began to be used as a generic contraction for are not, is not, etc., in early 19c. Cockney dialect of London; popularized by representations of this in Dickens, etc., which led to the word being banished from correct English.
Wiktionary
contraction 1 (context dialectal or informal English) am. 2 (context dialectal or informal English) are not, aren't; is not, isn't; am not. 3 (context dialectal informal English) have not, haven't; has not, hasn't, when used as an auxiliary.
Wikipedia
Ain't is a contraction for am not, is not, are not, has not, and have not in the common English language vernacular. In some dialects ain't is also used as a contraction of do not, does not, and did not. The development of ain't for the various forms of to be not, to have not, and to do not occurred independently, at different times. The usage of ain't for the forms of to be not was established by the mid-18th century, and for the forms of to have not by the early 19th century.
The usage of ain't is a perennial subject of controversy in English. Ain't is commonly used by many speakers in oral or informal settings, especially in certain regions and dialects. Its usage is often highly stigmatized, and it may be used as a marker of low socio-economic or regional status or education level. Its use is generally considered non-standard by dictionaries and style guides except when used for rhetorical effect, and it is rarely found in formal written works.
Usage examples of "ain't".
That was a whopping mean-looking snake that went across the path there, and I ain't going another step nearer the icehouse!
TNT shows the reruns every weekday afternoon (they call 'em the Afternoon Ponderosa Party, ain't that cute), and Seth just loves em.
In the den, Rory Calhoun yelled, 'You ain't goin nowhere, you sowbelly Yankee!
We got to ride on over to the Ponderosa, only this time you ain't never coming back.
He ain't been back since he left this mawnin', an' he told the cook he was eatin' out.
In fact, if you don't leave with Major Dillenger and me, you ain't going.
Hate to have to tell you that, you being a vegetarian, but ever since they legalized marijuana, it ain't the cash crop it used to be.
They wasn't much answer for a minute, only them kind of lookin' to and fro amongst themselves to see who might speak, and then somebody shouts out that no, damn it all, they ain't a-going to surrender nohow.