How did mille get so weird?

Question

The word mille is weird. In singular it is — or can be considered — an indeclinable adjective, and the main word is declined according to the grammatical role. In plural it is a declinable neuter noun, and the main word is always declined in plural genitive. If examples are needed, do ask, but I suspect any material on Latin grammar covers this.

How did this weird behaviour develop? Was mille originally only a noun (or only an adjective) and it was somehow split later on? Or were the singular and plural originally two different words which were later analyzed to be the singular and plural of the same thing? Does the word have cognates that might shed light on its origins? Do we know what lead to the mille/milia we know from classical Latin?

Answer 1

The etymology of mille is fairly clear, with cognates in other Indo-European languages. The singular and plural are definitely from the same etymological source. The Proto-Indo-European form would have been something like *smih₂-ĝʰsl-ih₂ "one thousand" (the exact formation is doubtful but the roots in question, "one" and "thousand", are not); cognates include Sanskrit sahásram and Greek χίλιος (from the second element only), both meaning "thousand".

As for the syntax, I wouldn't say it's that weird. Most Latin numerals are indeclinable, so the behavior of mille in the singular is nothing unusual. It can actually be a noun in the singular as well, e.g. mille passuum, with a noun in the partitive genitive. Plural thousands like tria milia just follow the normal rules for numerals with nouns -- i.e. the noun declines but the numeral does not -- except that in this case the noun happens to be a numeral itself, mille, which is then followed by the same partitive genitive as above.

Answer 2

I think that it is just a matter of irregular spelling, or possibly inaccurate transcription from different sources, some of which are hard to decipher, rather than anything really weird.

At Livy XXV.9 we find Cornelius lustrum condidit : censa sunt civium capita centum quadraginta tria millia septingenta quatuor. Livy appears to use either milia or millia at a whim.

Editors differ in their approach to Livy (and others!). Some 'correct' everything according to their own ideas : for instance, one may allow the recorded variations in the word, while others prefer one form only. The words septingenta and quatuor can appear as septinginta and quattuor. A brief Google search through different collections will show up this variability, though I really do not think that there is anything significant to be gained by pursuing the matter.

You might, in fact, ask a general question about how and why the accidence of cardinals differs. Unus, duo and tres are declinable. From quattuor to centum they are indeclinable. Hundreds upwards from ducenti to nongenti are declined as for -o and -a stems. Then comes the word in which you are interested. Mille (sometimes mile) is, as you say, usually regarded as an indeclinable adjective, but has a corresponding plural milia (or millia) which is declinable, as if a substantive of the third declension, similar to maria and cubilia, the plurals of mare and cubile. The fact that mille is thought of as an indeclinable singular adjective, while milia is undoubtedly a declinable plural noun is probably a fact of grammarians' usage, rather than unexplained divergent accidence : just as, in English, we have such expressions as many thousand miles and many thousands of miles — essentially the same, but used idiosyncratically — though we are content to accept either without a need to explain the reason for the difference.