I contend the word ain’t is a useful contraction of am not. Ain’t I correct in thinking that in the early 1800s, Bostonians thought of ain’t as an acceptable word without stigmatization?
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.
In other words, yes, ain't used to be the proper contraction for "am not", but because certain lower-class dialects began to use this word for pretty much any contraction of [be] or [has] with [not], it acquired a bad reputation.
This is still a very common colloquialism in the UK.