"How, then, was My rejection?" meaning My punishment; that is, it was severe. He warns them that there might descend upon them what descended upon those before them.
Muhammad said: The meaning of "Nakiri" (My rejection) is "Nakiriy," and the ya was elided because it is the end of the verse.
Surah Saba', verses 46 through 50.
﴿فَكَيْفَ كَانَ نَكِيريِ﴾ عقابي؛ أيْ: كَانَ شَدِيدا؛ يُحَذرهُمْ أَن ينزل بهم مَا نزل بهم.
قَالَ مُحَمَّد: (نَكِير) الْمَعْنى: نكيري، وحُذِفت الْيَاء؛ لِأَنَّهُ آخر آيَة.
سُورَة سبأ الْآيَات من الْآيَة ٤٦ حَتَّى الْآيَة ٥٠.