The index is the finger of Jupiter, and it takes that name from below rather than from itself. The Mount of Jupiter is the pad at its base, the planetary vocabulary was fixed on the mounts first, and the finger inherits what the mount was already called. Johannes ab Indagine printed the scheme in 1522 and it was old then, the Latin chiromantia of the twelfth century having carried it west; the French writers of the 1840s and 1850s kept it whole, and Cheiro gave it its popular English form.
What the tradition claims for the finger is what astrology claims for the planet. Jupiter signifies expansion, authority, confidence, appetite for office and the wish to lead, and those are read off a long, straight, well-set index: pride and a healthy self-regard in the favourable case, arrogance and a taste for command in the unfavourable one, a short or crooked index for the absence of both. The transfer is complete and it runs one way only. Nothing about the finger suggested Jupiter to anybody. The planet's meanings were assigned to a region of the hand and then read back out of it, which is the pattern of the whole system and the reason its parts agree with one another so neatly.
The Latin name is more honest about what the digit does. Index means the pointer, and pointing, whether to accuse, to instruct or merely to indicate, is the use every gesture tradition makes of it. That is a fact about the hand. The Jupiterian character is a fact about astrology.
Sources Introductiones apotelesmaticae elegantes in chyromantiam, physiognomiam, astrologiam naturalem, complexiones hominum, naturas planetarumJohannes ab Indagine (Johannes Rosenbach) with Palmistry for AllCheiro (William John Warner), The Earliest Chiromancy in the WestCharles S. F. Burnett