From The American Heritage Book of English Usage:
Ain't That A Shame?
"Ain't" -- its checkered past, its uncertain future
Ain’t is a word that ain’t had it easy. It first appeared in English in 1778, evolving from an earlier form an’t, which arose almost a century earlier as a contraction of are not and am not. In fact, ain’t seems to have arisen at the tail end of an era that saw the introduction of a number of our most common contractions, including don’t and won’t. Ain’t and some of these other contractions came under criticism in the 1700s for being inelegant and low-class, even though they had actually been used by upper-class speakers. But while don’t and won’t eventually became perfectly acceptable at all levels of speech and writing, ain’t was to receive a barrage of criticism in the 19th century for having no set sequence of words from which it can be contracted and for being “a vulgarism,” that is, a term used by the lower classes. At the same time ain’t’s uses were multiplying to include is not, has not, and have not. It may be that these extended uses helped provoke the negative reaction. Whatever the case, the criticism of ain’t by usage commentators and teachers has not subsided, and the use of ain’t has come to be regarded as a mark of ignorance. Use it at your peril.
But despite all the attempts to ban it, ain’t continues to appear in the speech of ordinary folks. Even educated and upper-class speakers see that ain’t has no substitute in fixed expressions like Say it ain’t so, You ain’t just whistlin’ Dixie, and You ain’t seen nothin’ yet.
ain’t I? The stigmatization of ain’t leaves us with no happy alternative for use in first-person questions. The widely used aren’t I?, though illogical, was found acceptable for use in speech by a majority of the Usage Panel in an early survey, but in writing there is no alternative to saying am I not?