The Quran root غ ل ظ gh-l-ẓ

فَبِمَا رَحْمَةٍ مِّنَ اللَّهِ لِنتَ لَهُمْ وَلَوْ كُنتَ فَظًّا غَلِيظَ الْقَلْبِ لَانفَضُّوا مِنْ حَوْلِكَ فَاعْفُ عَنْهُمْ وَاسْتَغْفِرْ لَهُمْ وَشَاوِرْهُمْ فِي الْأَمْرِ فَإِذَا عَزَمْتَ فَتَوَكَّلْ عَلَى اللَّهِ إِنَّ اللَّهَ يُحِبُّ الْمُتَوَكِّلِينَ ١٥٩

غَلِيظَ ghalīẓa · (and) harsh

So by mercy from Allāh, [O Muḥammad], you were lenient with them. And if you had been rude [in speech] and harsh in heart, they would have disbanded from about you. So pardon them and ask forgiveness for them and consult them in the matter. And when you have decided, then rely upon Allāh. Indeed, Allāh loves those who rely [upon Him]. (Surah Ali 'Imran, 3:159)

وَكَيْفَ تَأْخُذُونَهُ وَقَدْ أَفْضَىٰ بَعْضُكُمْ إِلَىٰ بَعْضٍ وَأَخَذْنَ مِنكُم مِّيثَاقًا غَلِيظًا ٢١

غَلِيظًا ghalīẓan · strong

And how could you take it while you have gone in unto each other and they have taken from you a solemn covenant? (Surah An-Nisa, 4:21)

يَا أَيُّهَا النَّبِيُّ جَاهِدِ الْكُفَّارَ وَالْمُنَافِقِينَ وَاغْلُظْ عَلَيْهِمْ وَمَأْوَاهُمْ جَهَنَّمُ وَبِئْسَ الْمَصِيرُ ٧٣

وَاغْلُظْ wa-ugh'luẓ · and be stern

O Prophet, fight against the disbelievers and the hypocrites and be harsh upon them. And their refuge is Hell, and wretched is the destination. (Surah At-Tawbah, 9:73)

يَا أَيُّهَا الَّذِينَ آمَنُوا قَاتِلُوا الَّذِينَ يَلُونَكُم مِّنَ الْكُفَّارِ وَلْيَجِدُوا فِيكُمْ غِلْظَةً وَاعْلَمُوا أَنَّ اللَّهَ مَعَ الْمُتَّقِينَ ١٢٣

غِلْظَةً ghil'ẓatan · harshness

O you who have believed, fight against those adjacent to you of the disbelievers and let them find in you harshness. And know that Allāh is with the righteous. (Surah At-Tawbah, 9:123)

مُّحَمَّدٌ رَّسُولُ اللَّهِ وَالَّذِينَ مَعَهُ أَشِدَّاءُ عَلَى الْكُفَّارِ رُحَمَاءُ بَيْنَهُمْ تَرَاهُمْ رُكَّعًا سُجَّدًا يَبْتَغُونَ فَضْلًا مِّنَ اللَّهِ وَرِضْوَانًا سِيمَاهُمْ فِي وُجُوهِهِم مِّنْ أَثَرِ السُّجُودِ ذَٰلِكَ مَثَلُهُمْ فِي التَّوْرَاةِ وَمَثَلُهُمْ فِي الْإِنجِيلِ كَزَرْعٍ أَخْرَجَ شَطْأَهُ فَآزَرَهُ فَاسْتَغْلَظَ فَاسْتَوَىٰ عَلَىٰ سُوقِهِ يُعْجِبُ الزُّرَّاعَ لِيَغِيظَ بِهِمُ الْكُفَّارَ وَعَدَ اللَّهُ الَّذِينَ آمَنُوا وَعَمِلُوا الصَّالِحَاتِ مِنْهُم مَّغْفِرَةً وَأَجْرًا عَظِيمًا ٢٩

فَاسْتَغْلَظَ fa-is'taghlaẓa · then it becomes thick

Muḥammad is the Messenger of Allāh; and those with him are forceful against the disbelievers, merciful among themselves. You see them bowing and prostrating [in prayer], seeking bounty from Allāh and [His] pleasure. Their sign is in their faces from the effect of prostration [i.e., prayer]. That is their description in the Torah. And their description in the Gospel is as a plant which produces its offshoots and strengthens them so they grow firm and stand upon their stalks, delighting the sowers - so that He [i.e., Allāh] may enrage by them the disbelievers. Allāh has promised those who believe and do righteous deeds among them forgiveness and a great reward. (Surah Al-Fath, 48:29)