What did it mean 2000 years ago ?
It was hyperbole, and basically just meant "disdain," in the sense of feeling someone was not worthy of your consideration or respect.
Let me show you an example from scripture that essentially proves it would likely have had to have been. The Greek word used in Luke 14:26 was μισεῖ, the same word used in the Greek OT of Genesis 29:31-33 several times:
31 And when the Lord God saw that Lea was hated, he opened her womb; but Rachel was barren. 32 And Lea conceived and bore a son to Jacob; and she called his name, Ruben; saying, Because the Lord has looked on my humiliation, and has given me a son, now then my husband will love me. 33 And she conceived again, and bore a second son to Jacob; and she said, Because the Lord has heard that I am hated, he has given to me this one also; and she called his name, Simeon. 34 And she conceived yet again, and bore a son, and said, In the present time my husband will be with me, for I have born him three sons; therefore she called his name, Levi. 35 And having conceived yet again, she bore a son, and said, Now yet again this time will I give thanks to the Lord; therefore she called his name, Judah; and ceased bearing.
Now, if we take this word to mean literal "hatred" as we understand the term today, what was Jacob doing being with her so many times that she bore four children to him? She was not a raging beauty, as verse 17 states, "Leah had no sparkle in her eyes, but Rachel was shapely and beautiful." So it wasn't because he thought she was so hot physically that he just decided, "What the heck, even though I hate her guts I'm gonna do it anyway," LoL. It likely meant he simply "disdained" Leah, because she was not his first choice and he got tricked into her.
The same Greek word was also often used in contexts that communicated
preference against someone. This is the context in which we find it in Luke 14:26, given that a parallel passage reads,
37 He who loves father or mother more than Me is not worthy of Me. And he who loves son or daughter more than Me is not worthy of Me. 38 And he who does not take his cross and follow after Me is not worthy of Me. (Matthew 10:37-38)
It is admittedly a tough question to answer convincingly given the unmistakably harsh meaning the word carries today. But I think context suggests it was more an expression communicating disdain than truly intense emotional hatred.
Note: See also Deuteronomy 21:15 and especially Deuteronomy 22:13-19, where the disdain the husband has for the wife is due to suspecting her of not being a virgin at the time of marriage.