Human beings have presumably, since time immemorial, used their fingers for pointing and listing, two primary modes of referring and organizing... Although the languages we have used in the last two or three millennia do not date back directly to Cro-Magnon times, it is still worthwhile noting that we use the term 'index' for both the pointing finger and a list of a certain kind that also figuratively points...
It got me thinking.
The library can broken down into two activities: organizing (technical services) and referring (reader services).
I visit a website and its homepage is called index.html. As I move my mouse, the pointer on the screen turns from an arrow to a hand with an outstretched index finger.
You can use your index finger to point out something to someone. Or you can bring it to your lips and make a shushing noise.