They have no knowledge of it, nor had their fathers. Grave is (18:5)

﴿مَّا لَهُم بِهِ مِنْ عِلْمٍ وَلَا لِآبَائِهِمْ ۚ كَبُرَتْ كَلِمَةً تَخْرُجُ مِنْ أَفْوَاهِهِمْ ۚ إِن يَقُولُونَ إِلَّا كَذِبًا﴾
[ الكهف: 5]

18:5 They have no knowledge of it, nor had their fathers. Grave is the word that comes out of their mouths; they speak not except a lie.

These fabricators have no knowledge of their claim that Allah has a son and nor did their fathers whom they followed in this regard have any knowledge.
This statement which has escaped their mouths without any understanding is heinous.
They only speak a lie which has no basis or support.

No knowledge have they of such a thing, nor had their fathers. Mighty is the word that comes out of their mouths [i.e. He begot (took) sons and daughters]. They utter nothing but a lie.

phonetic Transliteration

Ma lahum bihi min AAilmin wala liabaihim kaburat kalimatan takhruju min afwahihim in yaqooloona illa kathiban

Abdullah Yusuf Ali - Translation

No knowledge have they of such a thing, nor had their fathers. It is a grievous thing that issues from their mouths as a saying what they say is nothing but falsehood!

Safi-ur-Rahman al-Mubarakpuri

No knowledge have they of such a thing, nor had their fathers. Mighty is the word that comes out of their mouths. They utter nothing but a lie.

Page 294 English transliteration