ain't fell out of favor because it sounds very hillbillish

ain't = not it's not an abbrev'n

Wow that's very interesting - i didn't know that bit of history! You're right about the contraction uniqueness though - would help with people learning verb conjugations ;)

Kayleigh's answer is a good one. The funny thing, though, is that in the 1700's and 1800's, "ain't" was acceptable as correct English. It does actually serve a purpose that no other single word or contraction does. I'm not sure why it fell out of favor.

This is a slang word. It stands in place of "am not" or "is not" or "are not" For example "I ain't doing that" - I am not doing that "He ain't going to the park" - He is not going to the park "They ain't bothered"- They are not bothered I would not chose to use it - because it often makes people think that you are common and cannot speak English properly! But you may hear it a lot - especially on films or in rap music!