Virus is a neuter word of the second declension even though it ends on -us, as evidenced by its genitive on -i (it has no plural). Are there any other such words?

Bonus question: is it possible that there were two forms of virus, one masculine on -us, the other neuter on -um, that were later conflated into one word? Their origin might then be adjectival.

1 Answer 1


Tuomo Pekkanen's Ars Grammatica (a Latin grammar in Finnish) says that the second declension has three neuters ending in -us: vīrus, vulgus and pelagus. They are only used in the singular, and accusative is exactly like the nominative (not -um).

I have no clue about the origin of these words. I'm not sure if these words even have a similar history.

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    Pelagus is a Greek borrowing (τὸ πέλαγος, -εος) which retained its neuter gender; the Greek nom./acc. plural pelage does occur. Vulgus and virus are probably (thus Weiss) each a conflation of two nouns, a masculine and a neuter from the same stem and with the same meaning: in the case of vulgus, probably an s-stem neuter and an o-stem masculine; in that of virus, probably both o-stems (cognates: Gk. ἰός m., Ved. viṣam n.). Acc. vulgum occurs rarely.
    – TKR
    Commented Jul 4, 2016 at 1:10
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    @TKR, thanks! I asked a separate question about the plurals. You can answer about the plural declension of pelagus there if you want. You should also consider turning your comment into another answer here, but it's your call.
    – Joonas Ilmavirta
    Commented Jul 4, 2016 at 12:47
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    There's also cetus, which is another word from Greek (where it's neuter); my understanding is that the OLD calls it masculine in the singular and neuter in the plural (cete) but I don't have my OLD near me now to check. Commented Jul 4, 2016 at 23:48
  • @JoelDerfner: Cf. locus, sometimes plural loca.
    – Cerberus
    Commented Jul 5, 2016 at 16:44
  • @Cerberus, masculine and neuter plurals of locus have a different meaning.
    – Joonas Ilmavirta
    Commented Jul 5, 2016 at 20:10

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