remember when you used to get popped in the mouth for using the word "ain't..." followed by "ain't, ain't in the dictionary?" well... it is...

ain't: 1. nonstandard excapt in some dialects. am not; are not; is not. 2. nonstandard. have not; had not; do not; does not; did not. [ 1770-80; var. of amn't ( contrv. of am not ) by loss of m and raisng with compensatory lengthening of a; cf. aren't ]-usage. as a substitute for am not, and are not in declarative sentances, ain't is more common in uneducated speech than in educated, but it occurs with some frequency in the informal speech of educated, esp. in the southern and south-central states. this is especially true of the interrogative ain't i? used as a substitue for the fromal am i not? or aren't i? (considered by some to be ungrammatical) or for the akward 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 humorous or 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 effect or to represent speech. as a substitue for have not or has not and-occasionally in soutern speech-do not, does not, and did not, it is nonstandard except in similar humorous uses: you ain't heard nothin yet! see also aren't.

special thanx to random house webster's college dictionary.