ain't is colloquial for "am not" It is a very English word (you won't hear it in America) and can often be heard in the northern parts of the UK. "I ain't doing that" = I am not doing that. It is also used as a tag question as an alternative to isn't it? It's hot isn't it! OR It's hot ain't it? Tag questions seek affirmation for something the speaker already knows the answer to.

'Ain't'' is used to replace any of the following negatives: am not isn't aren't hasn't haven't For example, if you hear someone saying 'I ain't going', this means 'I am not going'. Please note that this is not good English. It's what we call 'non-standard', which means that less educated people use it in some varieties of English. You may hear 'ain't' in films and in popular music, but you should never copy this way of speaking.

it's a contraction for am not, is not, are not, has not, and have not

As my English teacher told me "There ain't no such word as 'ain't'!" Seriously there are some good answers underneath.