Deriving the Meaning of Monogenes
The key element to remember in deriving the meaning of monogenes is this: it is a compound term, combining monos, meaning only, with a second term. Often it is assumed that the second term is gennasthai/gennao, to give birth, to beget. But note that this family of terms has two nu's, rather than a single nu, found in monogenes. This indicates that the second term is not gennasthai but gignesthai/ginmai, and the noun form, genos. G. L. Prestige discusses the differences that arise from these two derivations in God in Patristic Thought (London: SPCK, 1952), 37-51, 135-141, 151-156.
Genos means "kind or type", ginomai is a verb of being. Hence the translations "one of a kind," "one and only," "of sole descent." Some scholars see the - genes element as having a minor impact upon the meaning of the term, and hence see monogenes as a strengthened form of monos, thereby translating it "alone," "unique," "incomparable."
An example of this usage from the LXX is found in Psalm 25:16,
"turn to me and be gracious to me,
for I am lonely (monogenes) and afflicted" ( NASB )
“By faith Abraham, when God tested him, offered Isaac as a sacrifice. He who received the promises was about to sacrifice his one and only son (monogenes).” Hebrews 11:17 NIV
Hence the preceding references clearly demonstrate that monogenes does not mean generation, that Christ was God's firstborn or first creation. Rather it speaks of uniqueness, that Jesus' shares a unique relationship with God as Father from all eternity. This is true of Christ alone since no other person has ever shared an eternal filial relationship with God.
From: The Forgotten Trinity [Minneapolis, MN, Bethany House Publishers, 1998], pp. 201-202, fn. 27)