And what about words that were not already defined in Scripture before the NT; e.g., regeneration, foreknowledge, propitiation, forerunner, etc., where were those definitions going to come from other than the Greek language?
Those are English translations of well known Greek words and-or compound words.
regeneration: παλιγγενεσια, παλιν+γενεσις (manifold LXX examples).
foreknowledge: προ+γνωσις (manifold LXX examples).
propitiation: ιλαστηριον, (Rom 3:25, Heb 9:5), Exo 25:17-22 (7 times), Exo 31:17, Exo 35:12, Exo 37:6-9 (3 times), Lev 16:2-15 (7 times), Num 7:89, Ezek 43:14-20 (5 times), Amos 9:1.
propitiation: ιλασμος, (1 John 2:2, 4:10), Lev 25:9, Num 5:8, Exo 30:10 (εξιλασμος, εκ+ιλασμος), Lev 23:27-28 (εξιλασμος, εκ+ιλασμος), and mixed forms at least ten more places in the LXX beyond the Torah.
forerunner: προδρομος, Num 13:20 LXX, Isa 28:4 LXX.
And whichever one in the Greek which the NT writer used is the meaning of "seeing" in that NT text.
As I have said, the context informs me of such things, and especially in the Testimony of the Messiah, so I will give one example containing two of the words for seeing and allow you to do the appropriate the research for yourself.
John 16:16-17 KJV
16 A little while, and ye shall not see
[θεωρεω] me: and again, a little while, and ye shall see
[οπτανομαι] me, because I go to the Father.
17 Then said some of his disciples among themselves, What is this that he saith unto us, A little while, and ye shall not see
[θεωρεω] me: and again, a little while, and ye shall see
[οπτανομαι] me: and, Because I go to the Father?