-um

Etymology 1

From Proto-Italic *-om, from Proto-Indo-European *-om (thematic masculine singular accusative and neuter singular nominative and accusative ending).

Alternative forms

• -om (conditioned variant used after -v-, -qu-, -gu-, -u- up through the middle of the first century AD)

Suffix

-um

1. inflection of -us:
   1. accusative masculine singular
   2. nominative/accusative/vocative neuter singular

Etymology 2

From Proto-Italic *-om, from Proto-Indo-European *-oHom (genitive plural ending).

Alternative forms

• -om (conditioned variant used after -v-, -qu-, -gu-, -u- up through the middle of the first century AD)
• -ûm, -ôm; -ūm, -ōm (alternative spellings formerly used in specific contexts in New Latin and modern editions of Old/Classical/Late Latin works; see usage notes)

Suffix

-um

1. genitive plural ending
   1. Found in -ōrum
   2. Found in -ārum
   3. genitive plural of -s

Usage notes

Latin genitive plural forms take the ending -um either by itself, or with additional preceding material (generally determined by the word's conjugation class). First and second declension nouns and adjectives usually have genitive plural forms ending in -ārum and -ōrum, but in some contexts take the short ending -um (without preceding -ār-/-ōr-) instead: this is common with words denoting weights, measures and monetary value and with distributive numerals.

• In the second declension, the genitive plural in -um can be found:
  • in distributive numerals: e.g. bīnus
  • in names of weights and measures: dēnārius, medimnum, nummus, sēstertius, stadium, talentum
  • in deus and dīvus (the latter having the archaic genitive plural form divom, spelled with O due to the preceding V)
  • in various other words, especially in poetry
• In the first declension, the genitive plural in -um can be found:
  • in the Greek-derived measure words amphora and drachma (less frequently than drachmārum)
  • in dactylic verse, in compounds of -cola and -gena
  • in dactylic verse, in some masculine Greek proper nouns, such as patronymics

In New Latin texts (i.e. from the Renaissance onwards), the genitive plural suffix -um was formerly often spelled with an accent as -ûm(or -ôm after V/U) when it appears instead of -ārum or -ōrum in words of the first or second declension such as amphorûm and deûm.

This circumflexed spelling -ûm was motivated at least in part by the reinterpretation of the ending as a contraction of -ōrum/-ārum. Because contraction generally produces a long vowel in Latin, this reanalysis resulted in an incorrect assumption that the vowel in the final syllable of genitive plural amphorum and deum was long by nature, and so distinguished in length from the naturally short vowel in the final syllable of the accusative singular form deum. Thus, the contraction hypothesis enabled second-declension genitive plural forms and accusative singular forms that would otherwise be spelled the same to be differentiated in a way that was imagined to correspond to a distinction in the natural length of the vowel. Compare the use of the circumflex to distinguish the nominative/vocative ending -a (with short ă) from the ablative ending -â (with long ā) in first declension singular nouns.

The contraction hypothesis also caused these genitive forms to be spelled with -ūm (or -ōm after V/U) in some older works by modern scholars that use macrons to mark long vowels, such as Lewis and Short's Latin Dictionary. However, the hypothesis of contraction and the corresponding assumption of a difference in vowel length is outdated: according to modern etymological understanding, all Latin words ending in -um, regardless of their case, number or declension, were pronounced in Classical Latin with a short vowel in the final syllable.

Third declension words that have genitive plurals ending in -um as an alternative to -ium have also been spelled with -ûm, again with the justification that the shorter ending is interpreted as being a contraction of the longer variant. Examples are coelestûm and caedûm used as spellings of the short genitive plurals of coelestis and caedes (compared to the long forms coelestium and caedium).

Aside from the contraction hypothesis, the use of the circumflexed spelling -ûm may additionally have been influenced by the use of the circumflex in the spelling of Greek genitive plural forms ending in -ῶν.