ain'tyoudaoicibaDictYouDict[ain't 词源字典]
1706, originally a contraction of am not, and in proper use with that sense until it began to be used as a generic contraction for are not, is not, etc., in early 19c. Cockney dialect of London; popularized by representations of this in Dickens, etc., which led to the word being banished from correct English.[ain't etymology, ain't origin, 英语词源]