• تَنكِصُونَ
• نَكَصَ

Appears 2 times in the Quran.

1. 8:48 وَإِذْ زَيَّنَ لَهُمُ ٱلشَّيْطَٰنُ أَعْمَٰلَهُمْ وَقَالَ لَا غَالِبَ لَكُمُ ٱلْيَوْمَ مِنَ ٱلنَّاسِ وَإِنِّى جَارٌ لَّكُمْ فَلَمَّا تَرَآءَتِ ٱلْفِئَتَانِ نَكَصَ عَلَىٰ عَقِبَيْهِ وَقَالَ إِنِّى بَرِىٓءٌ مِّنكُمْ إِنِّىٓ أَرَىٰ مَا لَا تَرَوْنَ إِنِّىٓ أَخَافُ ٱللَّهَ وَٱللَّهُ شَدِيدُ ٱلْعِقَابِ

   And when the Shaitan made their works fair seeming to them, and said: No one can overcome you this day, and surely I am your protector: but when the two parties came in sight of each other he turned upon his heels, and said: Surely I am clear of you, surely I see what you do not see, surely I fear Allah; and Allah is severe in requiting (evil).
   

2. 23:66 قَدْ كَانَتْ ءَايَٰتِى تُتْلَىٰ عَلَيْكُمْ فَكُنتُمْ عَلَىٰٓ أَعْقَٰبِكُمْ تَنكِصُونَ

   My communications were indeed recited to you, but you used to turn back on your heels,