What does (Ain't nothin' ) mean?
You are, my fire, The one, desire, Believe, when I say I want it that way
Tell me why, Ain't nothin' but a heartache,

What does (Ain't nothin' ) mean?

Question

What does (Ain't nothin' ) mean?
You are, my fire, The one, desire, Believe, when I say I want it that way 
Tell me why, Ain't nothin' but a heartache,

What does (Ain't nothin' ) mean?

Answer

It means "It isn't anything".

So it's like saying "It's nothing but a heartache".

Answer

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

Answer

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!''