Possible Clues:

• Neither fem. nor neut.
• Like most Sp. nouns that end in "o"
• Like some nouns, in Ger.
• Like Span. o-ending nouns
• Opposite of feminine: Abbr
• Counterpart of fem
• Gender expression label
• Like some Sp. nouns
• Gender abbr.
• Like 'el' or 'le': Abbr.