If someone accuses you of being stupid and you want to protest the accusation, this is an example of when you might say "I ain't stupid."
Origin of ain-t
- early assimilation, with lengthened and raised vowel, of amn't, contr. of am not; later confused with a'nt (are not), i'nt (is not), ha'nt (has not, have not)
From Webster's New World College Dictionary, 5th Edition
- From the earlier form an't, a contraction of am not, are not, and is not. The shift from IPA: /ænt/ to IPA: /eɪnt/ parallels a similar change in some dialects with can't. In other dialects the pronunciation shifted to IPA: /ɑːnt/, and the spelling aren't, when used to mean “am not”, is due to the fact that both words are pronounced IPA: /ɑːnt/ in some non-rhotic dialects. Historically, ain't was present in many dialects of the English language, but not in the southeastern England dialect that became the standard, where it is only found in the construction aren't I?.