Whence Invisible Backpack of *fill-in-the-blank* Privilege?

Now that I

a) am learning how to make simple, slide-show-based videos with "Windows Movie Maker" and

b) have a means to record my own voice-overs,

I'm thinking of making a series of slide-show based videos out of this post My own version of the Bipedal Privilege Backpack (each video focusing on two or three privileges at a time, and illustrated with stick-figure cartoons).

And I'd like to open with a discussion of what "Privilege" is, and what it means to have "an invisible backpack" full of it.

So I'm asking my circle:

What counts as "privilege"? Is privilege always founded in culturally determined biases? Is that the difference between "privilege" and "natural ability"?

and also: Whence the metaphor of the "invisible backpack"? Who thought that up, and what was the inspiration?