How to make sense of ζωὴν αἰώνιον? (John 3:36)

Question

I was reading the following verse, from John, and I came across the phrase ζωὴν αἰώνιον. This phrase looks odd to me, since ζωὴν is feminine, and at first glance, αἰώνιον would seem to be masculine or neuter.

The phrase means "eternal life", and of course it's an important phrase in John (and the Gospels in general) so I wanted to become more familiar with the adjective αἰώνιον, and to know why it's inflected in such a surprising way.

Why does it have the -ον ending? I would expect, instead, to see αἰωνίαν, so that the adjective would more clearly match the gender of the noun.

ὁ πιστεύων εἰς τὸν υἱὸν ἔχει ζωὴν αἰώνιον· ὁ δὲ ἀπειθῶν τῷ υἱῷ οὐκ ὄψεται ζωήν, ἀλλ’ ἡ ὀργὴ τοῦ θεοῦ μένει ἐπ’ αὐτόν.

He who believes in the Son has eternal life. He who disobeys the Son shall not see life, but the wrath of God remains upon him.

John 3:36

Answer 1

While most adjectives have three endings (e.g., masculine -ος, feminine -α/-η, neuter -ον), some adjectives have two endings. Such adjectives, which Smyth refers to as “adjectives of two endings,”¹ use the masculine ending for both the masculine and the feminine-gender declensions.

He provides other examples, including the adjectives ἄδικος, φρόνιμος, ἵλεως, etc.

That being said, sometimes αἰώνιος does decline in the traditional feminine ending -α.² Joseph Henry Thayer notes the following on his entry for αἰώνιος,³

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References

Smyth, Herbert Weir. A Greek Grammar for Colleges. New York: American Book, 1920.

Wilke, Christian Gottlob. A Greek-English Lexicon of the New Testament: Being Grimm Wilke’s Clavis Novi Testamenti. Trans. Thayer, Joseph Henry.Ed. Grimm, Carl Ludwig Wilibald. Rev. ed. New York: American Book, 1889.

Footnotes

¹ p. 74, §288
² cp. 2 Thes. 2:16: παράκλησιν αἰωνίαν
³ p. 20