To “heap fiery coals upon his head ” is a figure of speech drawn from the method of smelting metals in Bible times. Ore was put into a furnace, and a layer of coals was put not only underneath the ore but also on top of it.
فَٱلرَّدُّ بِٱلْمِثْلِ يَدُلُّ أَنَّنَا أَشْخَاصٌ مُجْتَرِئُونَ لِأَنَّ يَهْوَه يَقُولُ :‏ « لِي ٱلِٱنْتِقَامُ » .‏