فَكُّ

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

الْفَكُّ is the natural state of a definite noun and it is in this state ( مَرْفُوْعٌ) when the noun is: 1) the subject المُبْتَدَأُ or 2) the predicate الخَبَرُ or 3) اسْمُ كَانَ or 4) خَبَرُ إِنَّ or 5) الفَاعِلُ subject of a verb in the active voice or 6) نَائِبُ الفَاعِلِ subject of a verb in the passive voice or 7) it’s a noun of dependent declension a) نَعْتٌ / صِفَطٌ adjectives b) التَّوْكِيْدُ emphatic c) المَعْطُوْفٌ conjunction d) البَدَلُ synonym

الْفَكُّ loses it’s definite article, changing to فَكُّ due to it being a possessed noun

Freeing

To free (a prisoner or slave), rid of, be ceased, desist

Grammatical analysis of فَكُّ

اسْمٌ مَرْفُوْعٌ بِالضَّمَّةِ الزَّاهِرَةِ

Surah/Below is the only verse in the holy Quran containing the nounفَكُّ
Verse 
90-13the freeing of a neckفَكُّ رَقَبَةٍ