In another thread CJ said, "Ain't gonna happen", when talking about men becoming ballerinos.

"Ain't" is such a detested word here in the U.S. Did it come from "am I not?" Was it ever considered correct English? Or, has it always been incorrect?

Dictionary.com contains a usage note from the AHD which pretty well answers your questions.

There's also a scholarly treatise on the use of ain't at http://www.hfac.uh.edu/English/classes/GU4322/items/aint.html

"Ain't gonna happen" is one of those phrases, wrong or right, that has crept into our language. I notice it is the "ain't" that has you upset. Does that mean you accept the "gonna"?

Interesting. By the AHD usage note arnie cites, ain't never achieved the acceptance into standard English that was bestowed upon other contractions. But etymology on-line says that ain't, meaning "am not", "was in proper use until it began to be used as a generic contraction for are not, is not, etc., early 19c., which led to it being banished from correct English."

quote:

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I notice it is the "ain't" that has you upset. Does that mean you accept the "gonna"?

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Oh, no, Morgan, I am not upset at all. I know that phrase has crept into our language. It just made me think about whether ain't was ever completely acceptable.