Is there a connection between modus and (adverbial) modo?

Question

Lately I've come to dread coming across the word modo. Is it going to be an adverb meaning "just a moment ago" or "only (this and nothing more)", or a noun for "a way of doing something", "a musical measure", "a grammatical mood", "a unit of measurement", "a boundary", or some sense I've never heard of from the apparently limitless meanings of this word?

I suppose this minuscule suffering gives me a little empathy for people undergoing the troubles of learning English, which I was just reading about, where "just" shares some of the adverbial meanings, but also has an adjectival meaning related to justice--and not just "just" has this difficulty in English, but so do most common words, most infamously "set".

But…maybe that bit of empathic suffering could help me perceive modo more as Roman would. Often it helps to understand a primary meaning, which people understand the secondary meanings as extensions of, appropriately modified to fit whatever they're currently talking about. Is there some common or primary meaning here, in relation to which all the others make sense?

Answer 1

On the other hand, now that I've gotten access to de Vaan…

De Vaan points out that both modŏ and modō are attested, and suggests that the adverb did in fact originate from the noun. He derives modus from PIE *med- "measure out" (albeit with an unexpected vowel change), which would make "measure" the original meaning of the noun.

It also seems to have had the connotation of "limit": see the related moderāre "to set limits" (>moderate), modestus "limited" (>modest). So the adverb modõ "only" would come from the ablative singular, something like "by the limit". The connection between "only this and nothing more" and "limit" does make sense, and the shortening of final vowels is certainly attested: as sumelic points out, brevis brevians is a well-known phenomenon.

Answer 2

It seems unlikely that there's a connection between these two: if modo came from the ablative of modus (which is indeed a common source of adverbs), we would expect modō, not modŏ.

The most recent source I could find on this was the Rev. J. W. Donaldson, "On the Etymology of the Latin particle modo", in the Transactions of the Philological Society from 1854 (!). I'm sure there are more reliable and recent sources out there: in particular, De Vaan might have a much more reliable Proto-Indo-European origin for the adverb.

But Donaldson agrees with me:

With this [evidence] before us, we may as well take leave of the old etymology, as absolutely irreconcileable with the use of the adverb. I will only add, that if modŏ had in the slightest degree the sense of modō, Cicero would not have written (De Orat. ii.34.146.), "qui ea mediocriter modo consideraverit," nor would Sallust have composed such a sentence as the following (Cat. xxxix. 6.) : "Cujusque modi genus hominum, quod modo bello usui foret." And although there is no difference in the forms of the two words, it must not be forgotten, that while the last syllable of the ablative mŏdō is regularly and properly long, the usage of all the poets confirms the doctrine of Probus (p. 1424), and Festus (p. 140, Müller), that the particle mŏdŏ is a pyrrhichius [that is, two short syllables].

And he doubles down on this when responding to critics of his work:

A Member maintained that all the meanings of mŏdŏ could be deduced naturally from the original one of "by measure, only." If this can be done, it is a pity that it has not been done. All the attempts, with which I am acquainted, are utter failures […]

His suggested etymology is from Greek μοὶ δός or the Latin equivalent *mihi dō "give me" > *mīdō > modo, comparing it to cĕdŏ from *ce "here" + dō, and explaining the *ī > ŏ by comparison to *hī diē > hŏdiē "today". But I don't buy his analogy: surely *hī would be in the ablative, *hō, to agree with diē? He has other analogies, but all of them go back to PIE ablaut patterns in verbs.

Anyone with access to more recent scholarship is welcome to supplant this answer! I'm also interested to see if there's a strong PIE etymology for mŏdŏ.