A visual exploration of words that look the same, sound the same, or are otherwise easily confused. Updated weakly, on Sundaes, through 2016.
my guess is it is in the place of the 'o' in not. (like how do not becomes 'don't.) No idea why 'is' is replaced with 'ai' though.
Well, yeah, it would be in place of the O if it were a true contraction. But as you say, it's not short for "ai not." So is it just me, or couldn't we get along just spelling it "aint"?
And why "amn't" can't be used :(
Same goes for "won't" and "shan't". There must be meaning conferred by the apostrophe (i.e that it means "not"). Saves mix ups with "wont".
Here you go, http://en.wikipedia.org/wiki/Ain't