Meaning and usage of ain't

Sometimes I encounter ain't, but I really don't know how to translate it properly.

What does ain't stand for? If I really wanted to use it, in which contexts would you say it's acceptable using it?

Answer

Ain’t is a negative present-tense form of the verbs be and have employed in all persons and numbers:

I ain't           we ain't
you ain't         you ain't
he/she/it ain't   they ain't

It represents a coalescence of the ordinary spoken contractions of not and the three relevant forms of the two verbs:

am not    ⊲   a’n’t  )  
are not   ⊲   a’n’t  )  
is not    ⊲   i’n’t  )  ⊲ e’n’t/ha’n’t ⊲ orthographic ain’t/hain't 
have not  ⊲  ha’n’t  )  
has not   ⊲  ha’n’t  )

It is used wherever be not is used: as a copula, in progressive constructions, and in passives; and where have not is used as an auxiliary, in perfect constructions.

Ain’t is not slang (which means, roughly, a fairly novel usage employed by an ‘in-group’ as a token of their ‘in-ness’) but a colloquialism which was at one time used virtually universally. You find it very often in 18th- and 19th-century plays and novels, in the mouths of persons of high social standing.

However, it aroused particular hostility among 19th century schoolmarms, who assaulted it ruthlessly and succeeded in painting it as the mark of illiterate speech. Accordingly, it should not be used in formal contexts except as an ironic nod to the vernacular. It is still very common in speech, but regarded as sub-standard rather than merely non-standard.