"Ain't" ain't a word. My high school English teachers pounded that into my head. And they were right — "ain't" is not proper English. On the other hand, it is used colloquially by people all over the English-speaking world. Language is not just limited to those words found in reference books and textbooks. Whether formally recognized or not, utterances people use every day are language. So if you're working to improve your overall Japanese level, you might consider learning just a bit of what, strictly speaking, ain't proper Japanese.