The origin of "ain't" is a contraction of "I am not." It became less-than-fashionable when the lower classes began to generalize it's use. Same with the current use of the word "axe" instead of "ask" by the lower classes. "Axe" is actually the originial pronounciation, and it's switched a few times during it's use, but it's current use it by lower classes so it's being rejected by the majority of society.

And then, on the flipside, more and more upper class individuals are over correcting the "He and I" usage when it sometimes should be "He and me" on occassion, and so always saying "I" is fairly standard at this point.

Uh, anyway... This is me avoiding homework.