@witchfynder_finder @chr @shel "Thumb" apparently has roots in PIE meaning "to swell" and in Proto-Germanic is related to an adjective that just meant "thick".

"Index" is from Latin "indicare" with is "in/upon/on/into" + "proclaim" or, basically "to point out". They also used "index" to mean the finger you're thinking of among other things. So it looks like a straight-up borrowing. Looks like "index" was used to mean the forefinger in the 14th century, then fell out for the book thing, and "index finger" as a phrase popped back up in 1768.