ن ج سn-j-s
(س، ک) نَجِسَ يَنْجَسُ وَنَجُسَ يَنْجُسُ نَجَاسَةًnajisa yanjasu wa-najusa yanjusu najāsatan: to be impure. (This verb is not used in the Quran.)
نَجَسٌnajas (verbal noun, maṣdar, in the sense of an adjective): impure; using a verbal noun in place of an adjective is a well-known mode of intensification in Arabic (as if they were impurity itself), which is why it is singular although the noun it qualifies is plural. Only once in the Quran: ﴿اِنَّمَا الْمُشْرِكُوْنَ نَجَسٌ فَلَا يَقْرَبُوا الْمَسْجِدَ الْحَرَامَ بَعْدَ عَامِهِمْ هٰذَا﴾ (At-Tawbah 9:28)