The various contraction of the existential verb, "to be", are plentiful in English, but the one belonging to "I am not", seems either lost or dialectal, i.e. "ain't", though this word hasn't as sordid a past as grammatical pundits would have you believe.
During the 1620s the contraction "amn't" appeared in written speech, and probably existed far earlier than that, due to the ubiquity and usage of such forms. The word was eventually worn down to "an't" and then at last "ain't", this then coalesced with similar forms such as "haven't" becoming "han't" and then absorbed into "ain't" due to h-dropping, which still is a prominent feature of World English.
African American Vernacular English, a great provider of terms into the popular English lexicon, made the last addition to the evolution of "ain't", applying to it the sense of "did not".
This glorious chimera of existential and operative verbs would flourish in multiple negatives, which - again contrary to wide-held belief - was and still is natural feature of English. Even Geoffrey Chaucer, the much lauded innovator of English, used it.
Should we then accept "ain't" as a part of formal, academic English? It seems rather improbable due to the rigid standards of that particular variety of English, but otherwise say it ain't so, for this particular contraction. Hell, I an't done with talking about it, though!
(ADD STUFF ABOUT PREJUDICE CONCERNING THE CONTRACTION, ALSO ADD MORE SOURCES THAT ARE NOT WIKIPEDIA, SINCE THIS MAY BE VERY AD VERBIM)