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-o (2) Forms masculine adjectives and nouns.
Earlier *-u a collapse into a single ending, drawn variously from Latin -us, nominative masculine ending, -um, nominative neuter and accusative masculine and neuter ending. From Proto-Italic *-os and *-om respectively. From Proto-Indo-European *-o-s and *-o-m, thematic nominative and accusative endings.
For feminine nouns with this ending, see -o (3).