A contraction for are not and am not; also used for is not. [Colloq. or illiterate speech]. See An't.

Ain't is a contraction originally for "am not" and "are not", but now typically meaning "is not", "am not", "are not", "has not", or "have not". In some dialects it is also used as a contraction of "do not", "does not", and "did not", as in I ain't know that. The word is a perennial issue in English usage.