Each of the fingers has a name: thumb, index, middle finger, ring finger and little finger. But why are these names? Some names seem obvious; others are less so, but all must have reason, as in its origin with all the names that we apply to all beings
The Latin word "digitus" means "finger", appendix of extremities, and it is also a measure of length equal to about 18 millimeters. As we learn to count on the fingers, also it means "figure or number", from where on turn derives the current term "digital". See: http://www.antiquitatem.com/en/language-by-the-gesture-digital-computer .
The actual names of each of the fingers are a continuation of the Latin names and they are formed either from the position at hand, from some characteristic such as strength or size and from the role they can play.
St. Isidore of Seville sums up it perfectly in his Etymologies XI, 1, 70-71:
On man and his parts
The fingers (digitus) are so called, either because there are ten (decem) of the, or because they are connected handsomely (decenter), for the combine in them selves both the perfect number and the most appropriate order. The first finger is called thumb (pollex), vbecause among the rest it prevails (pollere) in strength and power . The sond is the index finger (index), which is also called “greeter” (salutaris) or pointer (demonsstratorius), because we greet someone (salutare) point something out (ostendere) usually with it. The third finger is called “immodest” (impudicus), because often an accusation of a shameful action is expressed by it.The fourth is the ring (anularis) finger, because it is the one on which the ring (annulus) is worn. It is also called medical (medicinalis), because physicians (medicus) use it to scoop up ground eye-salves. The fifth is called auricularis, because we use it to scrape out the ear (auris). (Translated by Stephen A. Barney et alii. Cambridge University Press)
De homine et partibus eius.
Digiti nuncupati, vel quia decem sunt, vel quia decenter existuntt. Nam habent in se et numerum perfectum et ordinem decentissimum. Primus pollex vocatus, eo quod inter caeteros polleat virtute, et potestate. Secundus index et salutaris, seu demonstratorius, quia fere eo salutamus, vel ostendimus. Tertius impudicus, quod plerumque per eum opprobrii insectatio exprimitur. Quartus anularis, eo quod in ipso anulus geritur. Idem et medicinalis, quod eo trita collyria a medicis colliguntur. Quintus auricularis, pro eo, quod eo aurem scalpimus.
It should draw attention to the curious and elementary etymologies of St. Isidore, following in the footsteps of some famous Roman grammarians as Varro.
So he says that they are called “digiti” (fingers) because they are “decem”, (ten) or because they coexist “decenter”, ("decently, properly") in an clearly unjustified exercise of imagination. The truth is that there is not an Indo-European word for "digitus", which seems rather a familiar form about which little can be said.
But it is correct the etymology of the first finger, pollex, (the thumb), which comes from pulgaris and this from pollicaris and this from "pollere", “to be strong”. In some places it is also called "flea dip" because so undesirable insects are killed with it, and there is for some people the explanation of why the current form is in Spanish "pulgar" (from “pulga”) and not "polgar" as it seems it should be derived from "pollicaris"; it would be a vulgar assimilation to “pulga” (flea). Rather the process however seems to have been the reverse: from pollicaris, it is deduced that pulgaris serves to kill the bugs.
The explanation about the second finger, "index" (salutaris, demonstrativus) also seems clear and obvious. For example, Horace have used it in their Sermons (or Satires). 2.8, 20-26:
Myself at top, then Viscus, and below
Was Varius: after us came Balatro,
Vibidius also, present at the treat
Unasked, as members of Maecenas' suite.
Porcius and Nomentanus last, and he,
Our host, who lay betwixt them, made the three:
Porcius the undermost, a witty droll,
Who makes you laugh by swallowing cheesecakes whole:
While Nomentanus' specialty was this,
To point things out that vulgar eyes might miss;
(Translation by John Conington, M.A. Corpus Professor of Latin in the University of Oxford).
'summus ego et prope me Viscus Thurinus et infra,
si memini, Varius; cum Seruilio Balatrone
Vibidius, quos Maecenas adduxerat umbras.
Nomentanus erat super ipsum, Porcius infra,
ridiculus totas semel absorbere placentas;
Nomentanus ad hoc, qui, siquid forte lateret,
indice monstraret digito;
The third finger is the "middle" "medius", or large (summus), as it is called in Latin. Why St. Isidorus and the Romans called it impudicus, infamis, famosus? St. Isidorus explains only half, although his readers know exactly what he means. It, 2400 years ago as today, was the ugly, obscene and insulting gesture of “giving the finger” or “flipping the bird”, “short the bird”, which so much extension has acquired in recent times. It consists on maintaining the four closed fingers, lift the middle on a clear representation of the penis and testicles; it is a vulgar gesture that we read in vulgar terms like "to fuck off". To this issue, see: http://www.antiquitatem.com/en/digitus-impudicus-giving-the-finger
The fourth is called “anularis”, from “anulus” ("ring") and also honestus and medicus in Latin, because in it the ring is worn as St. Isidore says.
"anulus" comes from Latin anus, meaning ring, in circular shape. It is clear, then, why it is named so to the "hole in the digestive tract ends and by which the excrement is expelled".
The name of this finger deserves its own article, because it is very much the ancient literature on the Roman rings, and on the fingers and hand in which it must to be carried. It is no less interesting to analyze why it is called "medical or doctor finger" St. Isidore gives us a simple explanation, which may not be enough. Aulus Gellius, Macrobius, Pliny give us curious and less interesting news on this finger. We also discuss it in a separate article.
The fifth finger is called in Latin "ultimus", "nimius" and "minimus" and meñique, "little", in Spanish precisely because it is the smallest. The Royal Spanish Academy explains the term as "Crossing menino, child, and mermellique or margarique, variations of margarite, from old French margariz, renegade, traitor, paper sometimes attributed to this finger on sayings and fables). In any case it involved with “minino, menino, and these with minimus, the smallest. On English is equally obvious "Little Finger" and in French "petit doigt", in German klein Finger, Italian mignolo, Catalan it petit, dedinho in Galician.
It is also called "auricularis", from “auricula”, the ear, by its adaptation to the function of picking ears.