The question of “what is a word” is not a matter of making appearances in prestigious dictionaries (which, by the way, ain’t has two, albeit small entries in the Oxford English Dictionary) but rather a matter of use. If even one English speaker says or writes ain’t, then ain’t is a word. And, thousands upon thousands of native English speakers do say ain’t on a daily basis. Even the stodgiest prescriptivist knows that ain’t is another way of saying isn’t, aren’t, am not, and have not. Ain’t is a word. So, all you prescriptive grammarians with vendettas against this “impurity” of the English language need to get over it and find some other “problem” to pick on.
History of Ain’t
But, why ain’t? The negative contraction ain’t most likely developed in the eighteenth century from amn’t and an’t, contracted forms of am not, as well as from the obsolete hain’t, a contracted form of has not and have not. After a vowel change and the loss of the glottal fricative [h], ain’t triumphantly arose as a multipurpose contraction. He is not coming, and he ain’t coming. We are not going, and we ain’t going. You have not seen anything yet, or you ain’t seen nothing yet.
Aren’t versus Ain’t
One of the reasons prescriptivists argue against the use of ain’t is the existence of more acceptable contractions. Is not becomes isn’t. (She is coming, isn’t she?) Are not becomes aren’t. (You are going, aren’t you?) Have not becomes haven’t. (You have brought it, haven’t you?) Has not becomes hasn’t. (He has returned it, hasn’t he?) And am not becomes aren’t? The argument for using standard forms instead of ain’t falls apart when we consider the contracted form of the first person singular present tense form of be and the negative adverb not. If ain’t ain’t a word, how then can aren’t be the contraction of am not?
The down and dirty answer is one of convention. Some grammarian a long time ago declared that the acceptable contracted form of am not is aren’t (I am right, aren’t I?), so aren’t become the accepted form. But, if we as English speakers were to follow our own convention, the contraction of am and not would be amn’t or even an’t, both of which are much closer to ain’t than aren’t. But, alas, I am still in the majority when it comes down to aren’t versus ain’t, aren’t I? So, although ain’t is a word (the OED confirms this fact) and aren’t illogically is the contraction of am not, know that ain’t ain’t made its way into the standard language just yet.
Oxford English Dictionary