Ain't had its origins in common English, but became increasingly stigmatized since the 19th century. See also amn't. Negative concord, popularly called "double negation", as in I didn't go nowhere; if the sentence is negative, all negatable forms are negated. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.