Oh shit - it's in Merriam-Webster Unabridged
Main Entry: ain't
Pronunciation: primarystressamacrnt
Variant(s): also an't \" also primarystressant or like AREN't\Etymology: probably contraction of are not, is not, am not, & have not
1 a : are not {you ain't going} {they ain't here} {things ain't what they used to be} b : is not {it ain't raining} {he's here, ain't he} c : am not {I ain't ready} -- though disapproved by many and more common in less educated speech, used orally in most parts of the United States by many cultivated speakers especially in the phrase ain't I
and Merriam-Webster Collegiate
Main Entry: ain't
Pronunciation: primarystressamacrnt
Etymology: contraction of are not
Date: 1749
1 : am not : are not : is not
2 : have not : has not
3 : do not : does not : did not -- used in some varieties of Black English
usage Although widely disapproved as nonstandard and more common in the habitual speech of the less educated, ain't in senses 1 and 2 is flourishing in American English. It is used in both speech and writing to catch attention and to gain emphasis {the wackiness of movies, once so deliciously amusing, ain't funny anymore -- Richard Schickel} {I am telling you--there ain't going to be any blackmail -- R. M. Nixon}. It is used especially in journalistic prose as part of a consistently informal style {the creative process ain't easy -- Mike Royko}. This informal ain't is commonly distinguished from habitual ain't by its frequent occurrence in fixed constructions and phrases {well--class it ain't -- Cleveland Amory} {for money? say it ain't so, Jimmy! -- Andy Rooney} {you ain't seen nothing yet} {that ain't hay} {two out of three ain't bad} {if it ain't broke, don't fix it}. In fiction ain't is used for purposes of characterization; in familiar correspondence it tends to be the mark of a warm personal friendship. It is also used for metrical reasons in popular songs {Ain't She Sweet} {It Ain't Necessarily So}. Our evidence shows British use to be much the same as American.