On the other hand, the NT does not actually speak of triunity. It is the relation of the only-begotten to the Father. So Jesus, the only-begotten Son, had a beginning to his life. And Almighty God can rightly be called his Begetter, or Father, in the same sense that an earthly father, like Abraham, begets a son. Nouns as the first part of the compound give the source, e.g., from Zeus, the earth.
Adverbs describe the nature of the derivation, e.g., noble or common.
This gives us the sense of only-begotten. The ref. is to the only child of one's parents, primarily in relation to them. But the word can also be used more generally without ref. In the NT Mono-genes occurs only in Lk, Jn. It is thus found only in later writings.