Based on what Beauflow said, "ain't" was considered a "real" word in medieval English and meant one thing and one thing only....a contraction for "am not". It was actually a word that was common with royalty. Then, the "commoners" got hold of it and started using it incorrectly "he ain't", "she ain't", "they ain't" etc, to where it became slang, what it is still considered today.

I am knowledgeable kinda-sorta of the origins of language...I guess slightly more than the average. "Q" was the last letter in the alphabet that was officially adopted in most places. Rather, people used "kw". In Old English "queen" was spelled "kwene". Also, it is said that "qu" is the result of an error in a major printing press in the very early days of such devices, where, when a "q" was printed, the press automatically, and erroneously for the time, automatically followed with a "u". Before they knew it, it was a standard part of the language.