• ٱسْتَغْلَظَ
• ٱغْلُظْ
• غِلْظَة
• غَلِيظ

Appears 13 times in the Quran.

1. 3:159 فَبِمَا رَحْمَةٍ مِّنَ ٱللَّهِ لِنتَ لَهُمْ وَلَوْ كُنتَ فَظًّا غَلِيظَ ٱلْقَلْبِ لَٱنفَضُّوا۟ مِنْ حَوْلِكَ فَٱعْفُ عَنْهُمْ وَٱسْتَغْفِرْ لَهُمْ وَشَاوِرْهُمْ فِى ٱلْأَمْرِ فَإِذَا عَزَمْتَ فَتَوَكَّلْ عَلَى ٱللَّهِ إِنَّ ٱللَّهَ يُحِبُّ ٱلْمُتَوَكِّلِينَ

   Thus it is due to mercy from Allah that you deal with them gently, and had you been rough, hard hearted, they would certainly have dispersed from around you; pardon them therefore and ask pardon for them, and take counsel with them in the affair; so when you have decided, then place your trust in Allah; surely Allah loves those who trust.
   

2. 4:21 وَكَيْفَ تَأْخُذُونَهُۥ وَقَدْ أَفْضَىٰ بَعْضُكُمْ إِلَىٰ بَعْضٍ وَأَخَذْنَ مِنكُم مِّيثَٰقًا غَلِيظًا

   And how can you take it when one of you has already gone in to the other and they have made with you a firm covenant?
   

3. 4:154 وَرَفَعْنَا فَوْقَهُمُ ٱلطُّورَ بِمِيثَٰقِهِمْ وَقُلْنَا لَهُمُ ٱدْخُلُوا۟ ٱلْبَابَ سُجَّدًا وَقُلْنَا لَهُمْ لَا تَعْدُوا۟ فِى ٱلسَّبْتِ وَأَخَذْنَا مِنْهُم مِّيثَٰقًا غَلِيظًا

   And We lifted the mountain (Sainai) over them at (the li taking of the covenant) and We said to them: Enter the door making obeisance; and We said to them: Do not exceed the limits of the Sabbath, and We made with them a firm covenant.
   

4. 9:73 يَٰٓأَيُّهَا ٱلنَّبِىُّ جَٰهِدِ ٱلْكُفَّارَ وَٱلْمُنَٰفِقِينَ وَٱغْلُظْ عَلَيْهِمْ وَمَأْوَىٰهُمْ جَهَنَّمُ وَبِئْسَ ٱلْمَصِيرُ

   O Prophet! strive hard against the unbelievers and the hypocrites and be unyielding to them; and their abode is hell, and evil is the destination.
   

5. 9:123 يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ قَٰتِلُوا۟ ٱلَّذِينَ يَلُونَكُم مِّنَ ٱلْكُفَّارِ وَلْيَجِدُوا۟ فِيكُمْ غِلْظَةً وَٱعْلَمُوٓا۟ أَنَّ ٱللَّهَ مَعَ ٱلْمُتَّقِينَ

   O you who believe! fight those of the unbelievers who are near to you and let them find in you hardness; and know that Allah is with those who guard (against evil).
   

6. 11:58 وَلَمَّا جَآءَ أَمْرُنَا نَجَّيْنَا هُودًا وَٱلَّذِينَ ءَامَنُوا۟ مَعَهُۥ بِرَحْمَةٍ مِّنَّا وَنَجَّيْنَٰهُم مِّنْ عَذَابٍ غَلِيظٍ

   And when Our decree came to pass, We delivered Hud and those who believed with him with mercy from Us, and We delivered them from a hard chastisement.
   

7. 14:17 يَتَجَرَّعُهُۥ وَلَا يَكَادُ يُسِيغُهُۥ وَيَأْتِيهِ ٱلْمَوْتُ مِن كُلِّ مَكَانٍ وَمَا هُوَ بِمَيِّتٍ وَمِن وَرَآئِهِۦ عَذَابٌ غَلِيظٌ

   He will drink it little by little and will not be able to swallow it agreeably, and death will come to him from every quarter, but he shall not die; and there shall be vehement chastisement before him.
   

8. 31:24 نُمَتِّعُهُمْ قَلِيلًا ثُمَّ نَضْطَرُّهُمْ إِلَىٰ عَذَابٍ غَلِيظٍ

   We give them to enjoy a little, then will We drive them to a severe chastisement.
   

9. 33:7 وَإِذْ أَخَذْنَا مِنَ ٱلنَّبِيِّۦنَ مِيثَٰقَهُمْ وَمِنكَ وَمِن نُّوحٍ وَإِبْرَٰهِيمَ وَمُوسَىٰ وَعِيسَى ٱبْنِ مَرْيَمَ وَأَخَذْنَا مِنْهُم مِّيثَٰقًا غَلِيظًا

   And when We made a covenant with the prophets and with you, and with Nuh and Ibrahim and Musa and Isa, son of Marium, and We made with them a strong covenant
   

10. 41:50 وَلَئِنْ أَذَقْنَٰهُ رَحْمَةً مِّنَّا مِنۢ بَعْدِ ضَرَّآءَ مَسَّتْهُ لَيَقُولَنَّ هَٰذَا لِى وَمَآ أَظُنُّ ٱلسَّاعَةَ قَآئِمَةً وَلَئِن رُّجِعْتُ إِلَىٰ رَبِّىٓ إِنَّ لِى عِندَهُۥ لَلْحُسْنَىٰ فَلَنُنَبِّئَنَّ ٱلَّذِينَ كَفَرُوا۟ بِمَا عَمِلُوا۟ وَلَنُذِيقَنَّهُم مِّنْ عَذَابٍ غَلِيظٍ

    And if We make him taste mercy from Us after distress that has touched him, he would most certainly say: This is of me, and I do not think the hour will come to pass, and if I am sent back to my Lord, I shall have with Him sure good; but We will most certainly inform those who disbelieved of what they did, and We will most certainly make them taste of hard chastisement.
    

11. 48:29 مُّحَمَّدٌ رَّسُولُ ٱللَّهِ وَٱلَّذِينَ مَعَهُۥٓ أَشِدَّآءُ عَلَى ٱلْكُفَّارِ رُحَمَآءُ بَيْنَهُمْ تَرَىٰهُمْ رُكَّعًا سُجَّدًا يَبْتَغُونَ فَضْلًا مِّنَ ٱللَّهِ وَرِضْوَٰنًا سِيمَاهُمْ فِى وُجُوهِهِم مِّنْ أَثَرِ ٱلسُّجُودِ ذَٰلِكَ مَثَلُهُمْ فِى ٱلتَّوْرَىٰةِ وَمَثَلُهُمْ فِى ٱلْإِنجِيلِ كَزَرْعٍ أَخْرَجَ شَطْـَٔهُۥ فَـَٔازَرَهُۥ فَٱسْتَغْلَظَ فَٱسْتَوَىٰ عَلَىٰ سُوقِهِۦ يُعْجِبُ ٱلزُّرَّاعَ لِيَغِيظَ بِهِمُ ٱلْكُفَّارَ وَعَدَ ٱللَّهُ ٱلَّذِينَ ءَامَنُوا۟ وَعَمِلُوا۟ ٱلصَّٰلِحَٰتِ مِنْهُم مَّغْفِرَةً وَأَجْرًا عَظِيمًۢا

    Muhammad is the Messenger of Allah, and those with him are firm of heart against the unbelievers, compassionate among themselves; you will see them bowing down, prostrating themselves, seeking grace from Allah and pleasure; their marks are in their faces because of the effect of prostration; that is their description in the Taurat and their description in the Injeel; like as seed-produce that puts forth its sprout, then strengthens it, so it becomes stout and stands firmly on its stem, delighting the sowers that He may enrage the unbelievers on account of them; Allah has promised those among them who believe and do good, forgiveness and a great reward.
    

12. 66:6 يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ قُوٓا۟ أَنفُسَكُمْ وَأَهْلِيكُمْ نَارًا وَقُودُهَا ٱلنَّاسُ وَٱلْحِجَارَةُ عَلَيْهَا مَلَٰٓئِكَةٌ غِلَاظٌ شِدَادٌ لَّا يَعْصُونَ ٱللَّهَ مَآ أَمَرَهُمْ وَيَفْعَلُونَ مَا يُؤْمَرُونَ

    O you who believe! save yourselves and your families from a fire whose fuel is men and stones; over it are angels stern and strong, they do not disobey Allah in what He commands them, and do as they are commanded.
    

13. 66:9 يَٰٓأَيُّهَا ٱلنَّبِىُّ جَٰهِدِ ٱلْكُفَّارَ وَٱلْمُنَٰفِقِينَ وَٱغْلُظْ عَلَيْهِمْ وَمَأْوَىٰهُمْ جَهَنَّمُ وَبِئْسَ ٱلْمَصِيرُ

    O Prophet! strive hard against the unbelievers and the hypocrites, and be hard against them; and their abode is hell; and evil is the resort.