你读一下下面 the freedictionary 对ain't 的意义及用法说明就可以了。

ain't (eɪnt)

1. Nonstandard except in some dialects. am not; are not; is not.

2. Nonstandard. have not; has not; do not; does not; did not.

[1770–80; variant of amn't (contraction of am not) by loss of m and raising with compensatory lengthening of a; compare aren ' t]

usage: As a substitute for am not, is not, and are not in declarative sentences, ain ' t is more common in uneducated speech than in educated, but it occurs with some frequency in the informal speech of the educated, esp. in the southern and south-central states. This is especially true of the interrogative ain't I? used as a substitute for the formal am I not? or for aren't I? (considered by some to be ungrammatical) or for the awkward amn't I? (which is rare in American speech). Some speakers avoid all of the preceding forms by substituting Isn't that so (true, the case)? ain ' t occurs in set phrases: Ain't it the truth! The word is also used for emphasis: That just ain't so! It does not appear in formal writing except for deliberate (often humorous) effect or to represent speech. As a substitute for have not or has not and - occasionally in Southern speech - do not, does not, and did not, it is nonstandard except in similar humorous uses: You ain't seen nothin' yet! See also aren't.