Why is DNA footprinting called "footprinting"?

I understand the technique but am just wondering how the name originated. "footprinting" means that there should be a visible print, but in DNA footprinting, the area where there are no bands (nothing to see) is called a footprint. Why? I understand that it corresponds to the portion that is attached to the protein and hence the nuclease can't act on it and all....but then why this name? It's name should be "invisible print" or something else like that....Am I not getting this right?