I believe index for the classical need to iterate through an array. E.g.

for (i = 0; I <= arr.length; i++) { var thing = arr[i] ... }

So to me it stands for "index" for array lookup.

Before map and iterators were implemented in a lot of languages, this was the defacto way to iterate a list. At least this is how I learned it in java/c back in the day. Nowadays I think most OOP languages including java have implemented the "for ... in ..." Syntax or similar which deprecates this convention.