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-o (3) Forms feminine nouns.
From Latin -us, feminine nominative ending. According to Penny (2002), the ending probably only survived in Old Spanish in mano as other modern Spanish feminine nouns ending in -o were learned additions drawn from Classical Latin. From Proto-Italic *-us 'id.' From Proto-Indo-European *-o-s, the thematic nominative ending, the same source of -o (2), which could be either masculine or feminine.