Tafsir of Al-Anbiya' 21:66-67

{ أف لكم ولما تعبدون من دون الله أفلا تعقلون }

"Fie" (uff) is a sound that, when uttered, signifies that the speaker is annoyed. He was annoyed by what he saw of their persistence in worshipping them after their excuses had been severed, and after the truth had become clear and falsehood had perished. Thus, he expressed his disgust (ta'affuf) toward them. The lam (in lakum) is to specify the object of this disgust; meaning: "This disgust is for you and for your gods."

{ قالوا حرقوه وانصروا ءالهتكم إن كنتم فاعلين * قلنا يانار كونى بردا وسلاما على إبراهيم * وأرادوا به كيدا فجعلناهم الا خسرين }