I grew up in West Virginia. My house was on a hill above a farm, the Potomac River surrounding it on three sides--which means Maryland on three sides, for all you folks who aren't intimately acquainted with mid-Atlantic geography. ;-) For the most part, people from my school, my town, had a pretty standard American accent and sound grammar. But we had our share of country accents around, too.
So I heard a lot of ain't over the years. And because I was apparently born with the grammar gene, acutely aware of what I did not want to sound like, and because my teachers taught me ain't was incorrect, I never used it. Well, except when making a point. Or trying to sound a certain way. But then it was a purposeful use of what I deemed something incorrect, so . . . ;-)
But ain't was used a lot back in ye olden days, so in my current work in progress, I had a few characters use it. Then I thought I'd better look it up to make sure it was in fact a contraction in use at the time. And I was pretty surprised with what I found.
Namely, that ain't began as a correct contraction for "am not" back in 1706. So it was perfectly fine to say "I ain't going." Use of it abounded, and all was well for a century or so.
Then people started using it for "are not" and "is not" . . . which was wrong. "You ain't what you seem" just didn't fly. This mis-use apparently started in London as part of the cockney accent, which Charles Dickens picked up on and immortalized. All of a sudden it's a mistake the English-speaking world over. One used so very mistakenly, and in ways that it's pretty hard to say "No, no, that one's wrong but this one's right," that it was banned from correct grammar altogether.
A rather funny life of a word, isn't it? I still ain't likely to use it much, even knowing its etymology and correct usage now--but you can bet if I do, I ain't going to use it the wrong way. ;-)