"And We already know that your breast is constrained by what they say," meaning by their saying that you are a sorcerer.
﴿وَلَقَدْ نَعْلَمُ أَنَّكَ يَضِيقُ صَدْرُكَ بِمَا يَقُولُونَ﴾ يَعْنِي بِقَوْلِهِمْ أَنَّكَ ساحرٌ،
