Is “Ain’t” a word that has undergone the process of specialization, pejoration, amelioration, or generalization?
Technically "ain't" is not a word, but the answer is generalization: it's a general reference to several phrases.
"Ain't is a contraction for am not, is not, are not, has not, and have not in the common English language vernacular. In some dialects ain't is also used as a contraction of do not, does not, and did not. The development of ain't for the various forms of to be not, to have not, and to do not occurred independently, at different times. The usage of ain't for the forms of to be not was established by the mid-18th century, and for the forms of to have not by the early 19th century." (Wikipedia)