a dialectal or nonstandard usage
Webster’s New World College Dictionary, 4th Edition. Copyright © 2010 by
Houghton Mifflin Harcourt. All rights reserved.
Word origin of 'ain't'early assimilation, with lengthened and raised vowel, of amn't, contr. of am not; later confused with a'nt (are not), i'nt (is not), ha'nt (has not, have not)