Ain't Definition

Am not.
Webster's New World
Is not or are not.
Webster's New World
Has not or have not.
Webster's New World
The definition of ain't is slang for the phrases am not, is not, are not and have not.
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."

(dialectal or informal) Are not, aren’t; is not, isn’t; am not.

Contraction of am not.
American Heritage
Used also as a contraction for are not, is not, has not, and have not.
American Heritage

Origin of Ain't

  • 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?.

    From Wiktionary

  • From the earlier form han't, a contraction of has not and have not. Han't shifted from IPA: /hænt/ to IPA: /heɪnt/, and underwent h-dropping in most dialects.

    From Wiktionary

  • 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