Ain't is a contraction originally used for "am not", but also used for "is not", "are not", "has not", or "have not" in the common vernacular (SLANG). In some dialects it is also used as a contraction of "do not", "does not", and "did not" (i.e. I ain't know that). The word is a perennial issue in English usage. It is a word that is widely used by many people, but its use is commonly considered to be improper.
Related words and usage
Ain't was preceded by an't, which had been common for about a century previously. An't appears first in print in the work of English Restoration playwrights: it is seen first in 1695, when William Congreve wrote I can hear you farther off, I an't deaf, suggesting that the form was in the beginning a contraction of "am not". But as early as 1696 Sir John Vanbrugh uses the form for "are not": These shoes an't ugly, but they don't fit me. At least in some dialects, an't is likely to have been pronounced like ain't, and thus the appearance of ain't is more a clarified spelling than a new verb form. The related word hain't is an archaic and non-standard contraction meaning has not or have not. It can be found in literature, particularly in Mark Twain's stories such as The Adventures of Tom Sawyer. It is reminiscent of hae (have) in Lowland Scots. Another old non-standard form is baint or bain't, apparently a contraction of "be not".