وَإِنْ كَانَ آخِرُ الِاسْمِ يَاءً مَكْسُورًا مَا قَبْلَهَا، أَدْغَمْتَ الْيَاءَ فِي الْيَاءِ، وَفَتَحْتَ الْيَاءَ الثَّانِيَةَ؛ لِئَلَّا يَلْتَقِيَ السَّاكِنَانِ، تَقُولُ فِي قَاضٍ: «قَاضِيَّ».
Translation: If the noun ends with yāʾ preceded by kasrah, assimilate that yāʾ into the possessive yāʾ and give the second yāʾ fatḥah so two silent consonants do not meet; thus qāḍin yields qāḍiyya.
Here idghām means bringing the two yāʾs together as a single geminated yāʾ.
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Iḍāfah to Yāʾ al-Mutakallim and the Six Nouns
وَإِنْ كَانَ آخِرُ الِاسْمِ يَاءً مَكْسُورًا مَا قَبْلَهَا، أَدْغَمْتَ الْيَاءَ فِي الْيَاءِ، وَفَتَحْتَ الْيَاءَ الثَّانِيَةَ؛ لِئَلَّا يَلْتَقِيَ السَّاكِنَانِ، تَقُولُ فِي قَاضٍ: «قَاضِيَّ».
Translation: If the noun ends with yāʾ preceded by kasrah, assimilate that yāʾ into the possessive yāʾ and give the second yāʾ fatḥah so two silent consonants do not meet; thus qāḍin yields qāḍiyya.
Here idghām means bringing the two yāʾs together as a single geminated yāʾ.