Why is 1/12 called an "uncia" in Latin?

Question

The Latin word uncia (which is the origin of the English word ounce) denotes ¹/₁₂ of a pound. Does anybody know the etymology of this word?

Shouldn’t it be something more like *docia, or anything starting with a d? Uncia sounds more like ¹/₁₁.

Answer 1

All etymological dictionaries (Walde, Meillet) say that uncia is derived from Proto-Indo-European *oi-nos, "one", the same stem as Latin unus, "one", and English one/an. (This stem at some point replaced in many languages the older stem used for "one", sem-, as reflected in English (through Latin) simple, single, simultaneous, etc.) Walde on uncia suggests that its original meaning was "unit":

...als *oin(i)cia "Einheit" zu ūnus...

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So how did a word related to "one" come to mean "one twelfth"? Meillet doesn't know:

Le nom de l'unité fractionnelle est évidemment dérivé de ūnus ; et tous les autres s'y rattachent. Il s'agit de termes techniques dont la formation est singulière.

So it was an idiomatic development in older Latin. Because uncia was mainly the smallest unit of weight in early Republican currency, perhaps the smallest unit was simply called "one-ness" or "unit".

It just so happened that an uncia was taken as 1/12th of an as, which was the central coin of the new Roman system introduced ca. 280 BC and based on the older system of the aes grave ("signed bronze"), where one unit of aes grave equalled twelve unciae, small "pellets" of bronze. Note that the weight represented by the uncia was also an important quantity that was often used, so uncia and as were the two main coins, and also the smallest and the largest, respectively. As a unit of weight, uncia was also used outside coins. Coins like the denarius, worth ten asses, were only introduced later; the word de-n-arius simply means something like "tenner" (cf. English de-cimal).

However, an uncia was also one twelfth of a pes, a foot, like the English word inch, no doubt related. I don't know whether the unit of length or the unit of weight (from which English ounce) was older, so I don't know which influenced which.

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There were many duodecimal numeric systems in use around the Mediterranean, so it seems likely that the uncia was at some point equalled to the smallest "unit" in any duodecimal system. I believe duodecimal systems are ultimately related to sexagesimal systems (=5x12) as used in ancient Mesopotamia. There are more ways you can divide twelve into integers than, say, ten: you can divide twelve by 2, 3, 4, and 6, while ten can only be divided by 2 and 5, so twelve as a base was more useful in many simple physical transactions and quick calculations. Remnants of such a system in modern cultures are the way our hours and minutes are 60 times the smaller unit, and the way we use sexagesimal minutes and seconds in angles (360°=6x60). And of course the fact that we have twelve hours in a day.

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I can only hypothesise about the origin of the root un- "one" in counting, but Wikipedia on Sexagesimal offers this interesting bit of information:

It is possible for people to count on their fingers to 12 using one hand only, with the thumb pointing to each finger bone on the four fingers in turn. A traditional counting system still in use in many regions of Asia works in this way, and could help to explain the occurrence of numeral systems based on 12 and 60 besides those based on 10, 20 and 5. In this system, the one (usually right) hand counts repeatedly to 12, displaying the number of iterations on the other (usually left), until five dozens, i. e. the 60, are full.

It just so happens that a Latin word *uncia (apparently unattested but noted by Meyer-Lübke) for "finger joint" is mentioned in Meillet and Walde. It is mentioned under and apparently derived from uncus, "curved". Perhaps the root *un(i)(c)- for "one" originated in this practice of counting by finger joint, where you have twelve finger joints in the right hand for the thumb to touch?

Note that uncus is apparently related to unguis "claw" (v. Meillet) and to English ungulate, anchor, and angle (all from Latin), and to English "nail" (from Proto-Germanic, v. Walde). The Proto-Indo-European root was apparently something like *-nk-, "bend" (Oxford English Dictionary); in some languages, a "vulgar" vowel like u- was prefixed (Meillet: "populaire"), resulting in uncus etc.

If uncia is related to uncus, then the precursor of unus "one" must be derived from uncia; but then this must have happened before the Italic and Germanic branches of Proto-Indo-European diverged, since the Germanic languages also have words for "one" based on this Proto-Indo-European precursor of unus. It seems unlikely that both uncus and uncia should have survived through the Proto-Gallo-Italic phase into Latin for so long and made it unscathed, especially considering that unus would still have had to drop its -k- sound! Therefore, if uncia "1/12" should be related to uncus "curved", it would have to be unrelated to unus/one; but the relation between unus and uncia appears to be undisputed, so it seems that uncus and uncia are not related. Perhaps there was some sort of folk etymology at work in the archaic age, where speakers or writers mixed up some of those words and their etymologies?