مَّا لَهُم بِهِۦ مِنْ عِلْمٍ وَلَا لِـَٔابَآئِهِمْ ۚ كَبُرَتْ كَلِمَةً تَخْرُجُ مِنْ أَفْوَٰهِهِمْ ۚ إِن يَقُولُونَ إِلَّا كَذِبًا
Maa lahum bihee min 'ilminw wa laa li aabaaa'ihim; kaburat kalimatan takhruju min afwaahihim; iny yaqooloona illaa kazibaa
They have no knowledge about this, nor did their forefathers––it is a monstrous assertion that comes out of their mouths: what they say is nothing but lies.
They have no knowledge of this, nor did their forefathers. Monstrous is the word that comes out of their mouths — they speak nothing but a lie.
The charge here is about inherited speech: a claim passed down through generations and never examined by anyone who repeated it. Arabic marks its size with kaburat, a verb for physical enormity, so the weight falls on the phrase itself as much as on the people saying it. Surah al-Kahf opens by praising a Book with no crookedness in it, and this is the crooked word set against that.