What is meant by "nor to their fathers" in the verse: (They have no knowledge of it, nor had their fathers. Grievous is the word that comes out of their mouths...) and does it mean that the apostate's father has no knowledge because of his son's apostasy?

The verses (Al-Kahf 1-5) are interpreted as Allah's warning to the Jews, Christians, and polytheists who claimed that He had a son. This is because their saying is not based on knowledge from them or from their forefathers; rather, it is a pure lie of immense vileness and ugliness. Allah has gradually refuted this saying: first, by negating knowledge from its proponents and their forefathers; second, by describing it as vile; and third, by stating that it is a lie contrary to truth. The forefathers intended in the verse are those who uttered this corrupt statement, not the monotheist forefathers, and every person is held accountable for their own actions, not for the actions of others.