Who is Fanny?

Classic Question

While raking leafs I was listening to some tunes and, "The Weight" by The Band came on. Well, that song is just perfect but who is Fanny and why does she have to take a load off? Can anyone shed any light on this subject? I mean, thousands of words have been written about who is "So Vain", and how "Sweet Caroline" was, but no Fanny.