"Why then"—meaning why did not—"when you heard it, the believing men and believing women think good of themselves"—that is, of their brothers—"and say: This is a clear falsehood"—meaning a manifest lie.
﴿لَوْلَا﴾ هَلَّا ﴿إِذْ سَمِعْتُمُوهُ ظَنَّ الْمُؤْمِنُونَ وَالْمُؤْمِنَات بِأَنْفسِهِم﴾ أَيْ: بِإِخْوَانِهِمْ ﴿خَيْرًا وَقَالُوا هَذَا إفْك﴾ كذب ﴿مُبين﴾ بَين