I know "ain't" is an informal expression. But what's the origin of it?

Rebecca

The phrase "ain't" started in the 1700s when it was written as "amn't" as a contraction for "am not" until the 1800s. That's when "amn't" developed into "ain't" and started being used and became a generic contraction for "is not," "has not," "are not," and so on. It was then ruled as improper English when it was used by certain dialects. Ex: I ain't seen anything like it before. Ex: They ain't going anywhere tonight.