The etymology of "ain't"

As I was growing up and progressing through the educational system, the word "ain't" was appropriately categorized as an "unforgivable linguistic sin," and thus I never had reason to wonder about its origin. This week I encountered its history while reading Sister Bernadette's Barking Dog, a book about the "quirky history and lost art of diagramming sentences."

The English language has contractions for "are not" (aren't) and "is not" (isn't) but nothing for "am not." The logical "amn't" is a word that children create as a neologism, and one that was used by James Joyce, but is not routinely used except apparently in some parts of Ireland and Scoland. "Ain't" was created to fill that linguistic void as the contraction of "am not."

Therefore, strictly speaking, "he ain't" and "you ain't" are grammatically incorrect, but "I ain't" is grammatically o.k. as shorthand for "I am not," but it's not conventionally accepted. What we use instead is "aren't," as in "I'm right, aren't I?" However, "aren't" is really the contraction of "are I not," which is of course grammatically totally wrong.

Addendum: I thought it might finally be possible to write a blog entry without reference to Wikipedia, but a glance there reveals stuff I should put in for those wordmongers who visit this blog. "An't" was a precursor among Restoration playwrights. Related words include "haint" and "baint." The word "ain't" has been famously used by the Cowardly Lion in Wizard of Oz, by Gershwin for the libretto of Porgy and Bess, by Al Jolson in The Jazz Singer, and by a young baseball fan speaking to Shoeless Joe Jackson.

Addendum 2014: I recently noticed several uses of "an't" in Charles Dickens' The Pickwick Papers, written several centuries after the Restoration playwrights mentioned above.

• "Bless your innocence, sir, that an't it," replied Sam... (Chapter 16)
• "An't the gentleman a shot, sir?" inquired the long gamekeeper. (Chapter 19)
• "Wery good thing is weal-pie when you know the lady as made it, and is quite sure it an't kittens..." (Mr. Weller senior speaking, Chapter 19)

I note that in these instances it is not being used as a contraction of "am not."  I also found an instance of the usage of "ain't" in Chapter 38 (by Sam Weller).