Ain’t, is a slang word, used in a similar way to ‘isn’t’.
The first known recording of it was in 1706 where it was considered quite proper. It was a contraction of ‘am not’ and would have been present in the vocabulary of the aristocracy at the time. However, in the early 1800s, it began to also be used to replace ‘are not’, ‘is not’,’has not’ and more. It was popularised in this manner by Dickens as he attempted to capture the cockney dialect, after that, it fell out of favour with the upper classes who didn’t want to be associated with Dickensian riffraff, and it is now considered poor English.