مَسَدٍ

A word in the Arabic language will either be 1) a verb or 2) a noun or 3) a particle – مَسَدٍ is a noun

A noun may either be 1) Definite or indefinite 2) Masculine or feminine 3) Singular, dual or plural 4) Nominative, accusative or genitive

مَسَدٌ is the natural state of this indefinite noun and it becomes مَسَدٍ due to: 1) a preposition that precedes it or 2) a possessed noun that precedes it or 3) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

To twist a fibre of a cord strongly

Grammatical analysis of مَسَدٍ
اسْمٌ مَجْرُوْرٌ بِالكَسْرَةِ الزَّاهِرَةِ 
نَكِرَةٌمُفْرَدٌمُذَكَّرٌمَجْرُوْرٌم س دبَاب نَصَرَمَسَدَ يَمْسُدُفَعَلٌاسْمٌاِسْمُ الجِنْسِ
IndefiniteSingularMasculineGenitiveRootTwisted fiberنPatternNounGeneric or common Noun
twisted strands
Surah/Below is the only verse in the holy Quran containing the genitive, indefinite nounمَسَدٍ
Verse
111-5In her neck a rope of twisted fiberفِيْ جِيْدِهَا حَبْلٌ مِّنْ مَّسَدٍ