When did "ain't" become slang?

According to the following sources:

Ain't was originally the proper contraction for am not. The contraction became popular as a generic one in the 19th century and started to be considered nonstandard ( not slang) English since then:

• 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.

Source: Etymonline

Ain't:

• Historically, “ain’t” appears to have evolved in the late 1700s from an earlier > - form, “an’t,” which was first recorded in the late 1600s but was probably common in speech early in that century. So let’s start with the earlier form.

• “An’t” was originally a contraction of “am not” (as in “I an’t going”) and “are not” (as in “you an’t” or “we an’t” or “they an’t”). The earliest published citations are 1695 (for “an’t” = “am not”) and 1696 (for “an’t” = “are not”).

• But as early as 1710, “an’t” was also being used in place of “isn’t” as a contraction for “is not,” as in “that an’t fair,” or “he an’t here.”

  • These citations come from Merriam-Webster’s Dictionary of English Usage, and are earlier than those in the Oxford English Dictionary.

  • As for correctness, the OED labels “an’t” = “are not” simply as a contraction (presumably as correct as, say, “isn’t” or “don’t”). But it labels “an’t” = “am not” as a colloquial usage; that is to say, it was somewhat less correct. And finally the OED labels “an’t” = “is not” as an illiterate usage.

• Thus, so far we have (1) the legitimate “an’t” – a contraction of “are not”; (2) the colloquial “an’t” – a contraction of “am not”; and (3) the illiterate “an’t” – a contraction for “is not.”

• Strictly speaking, by the way, the contraction for “am not” should be “a’n’t” or “amn’t,” and in fact people once used “amn’t”; they still do in Irish English and Scots English.

• Meanwhile, in the late 1700s people began spelling “an’t” as “ain’t,” which may have been closer to the way it was pronounced at the time. (A common pronunciation of “are,” for example, was “air.”) Before long, “ain’t” became the usual spelling, and by the late 1800s, “an’t” had disappeared. What disappeared along with it were any claims that “ain’t” may have had to being a legitimate contraction.

• Keep in mind that “an’t” came along at a time when a large family of English contractions was being formed: words like “don’t,” “won’t,” “can’t,” “isn’t,” and many more that we now consider standard English.

• “An’t” (and later “ain’t”) was just one of the crowd for many years, and was used by the upper classes as well as the lower, educated and otherwise. You see it in a lot of late 18th-century and early 19th-century English novels in the mouths of ladies and gentlemen.

• But “ain’t” was different from the rest, and in the 19th century, criticisms arose. The other contractions seemed to have a clearly traceable parentage, while “ain’t” was all over the place. It just wasn’t as clear in its derivation as a word like “don’t” (do not), or “can’t” (cannot), or “won’t” (will not).

• And to complicate the picture even further, uses of “ain’t” started multiplying, so that it was used as a contraction of “has not” (as in “he ain’t been here”) and “have not” (as in “we ain’t seen him”). Chaos!

• For all these reasons, since the 19th century “ain’t” hasn’t been considered a legitimate contraction and is still described in dictionaries as “nonstandard.” But it does live on, and probably always will. When educated people use it now, though, they probably intend a kind of reverse snobbery or are trying for a humorous effect.

Source: www.grammarphobia.com