Yes, Alyosha, it's slang. 'ain't' means 'isn't' EXAMPLES: There ain't no beer left. ('ain't' is often used in a double negative) Ain't it nice? Some people think it's common and shouldn't be used. In British English, however, it's used in a lot of dialects - but isn't accepted as intelligent conversation. Personally, I don't mind it (but I wouldn't teach my students to use it). Dialects are dialects and they're allowed to be different from the standard use. You're right - 'ain't no denying' means 'there's no doubt, it's absolutely that way'. We often use 'ain't' in song lyrics: "Ain't no sunshine when she's gone...." As a teacher I'd say be aware of it so you understand it when you hear it, but it's probably best not to get into the habit of using it yourself.

Yes, the word "ain't" is American slang. It means is not, are not, am not, is no, are no, etc.. Ain't no denying means "there is no denying".