It means "It isn't anything". So it's like saying "It's nothing but a heartache".

It means they want to write a popular song without regard to English grammar.

Ain't isn't a real word. It's slang that is often used by teens and adults but, it's not a real word. Nothin' isn't a real word either. To answer your question thouain't Ain't means isn't, or am not. And nothin' means nothing. So ain't nothing would mean isn't nothing or a full sentence would be. ''I ain't a fool.'' or ''It ain't what it looks like!''