Plural Form of Words Ending in -us - History

History

The -us singular form with an -i plural comes from Latin. However, the morphology of Latin nouns is complex and not every Latin word ending in -us pluralized in -i. The ones that did largely were from the second declension masculine. Briefly, a declension is the way a noun changes to reflect facts about the object to which it refers (e.g., its gender or number) or the relationship that the noun has to other words in the sentence. Remnants of the Old English declension system can be seen in words like I, me, we, and us in modern English, as well as (more distantly) in the 's enclitic. In Latin, just as in many languages spoken today, a word having several forms is the rule rather than the exception. Most words indicate their declension (including their number, i.e. whether they are singular or plural) using an affix, much as we use 's today to indicate possession and (usually) -s to indicate plurality. Specifically, the nominative singular form of second declension masculine nouns is marked with -us, and the nominative plural with -i. More at Latin grammar. Confusion arises because some Latin words ending in -us would not have pluralized with -i. Examples of regularly declined nouns include third declension neuters, such as opus, with the plural opera, and fourth declension masculine and feminine, such as sinus and tribus, with plurals sinūs and tribūs. Some idiosyncratic instances are bus, a curtailed form of omnibus, a dative plural, and ignoramus, a verb.