Ain't (ain't), isn't time this "word" is accepted?

We're told from preschool & upward by teachers, that aint is not a word, it can't be found in dict.s and it's improper grammar to use in a sentence. This is all nonsense, of coarse aint is a word it has meaning from the speaker to listener, I found it in most dict.s, and there's nothing in grammar that forbids aint or any other word for that matter. Aint goes back to the 1600s. What wong w/it?