From the triliteral root ن-س-ي (n-s-y), which carries the core meaning of forgetting or leaving something behind. The same root appears in the noun نِسْيَان (nisyān, "forgetfulness") and in the Quranic phrase لَا تَنْسَ نَصِيبَكَ ("do not forget your share"). Cognates appear in other Semitic languages; cf. Hebrew נָשָׁה (nāšāh, to forget).

In Arabic-speaking cultures, forgetting something — especially a promise or a person's kindness — carries a mild moral weight. The phrase "الناس تنسى لكن الله لا ينسى" ("People forget, but God does not forget") is a common folk saying offering comfort. Be careful not to confuse نَسِيَ (nasiya, to forget) with نَسَأَ (nasa'a, to postpone), which sounds similar but is a different root entirely.