it IS a real word. its a contraction as someone already said. just like you wouldnt wear shorts an t-shirt if you were meeting the queen, there are levels of language at diffrent acceptabilitys. like i said aint IS a word. its just non-standard so wouldnt be used in an essay or shouldnt be used in a formal speech. there are some people who argue that slang is bad, non-standard and therefore shouldnt be used, but thats almost a diffrent matter.