against Mary a great slander,)

The great slander is the great lie they invented about Mary the chaste when they accused her in her chastity and said about her what cannot be repeated or accepted. Mary the chaste had a very pure life, she has grown up in the prayer chamber before she conceived the Prophet Issa (PBUH):

 فَتَقَبَّلَهَا رَبُّهَا بِقَبُولٍ حَسَنٍ وَأَنبَتَهَا نَبَاتًا حَسَنًا وَكَفَّلَهَا زَكَرِيَّا ۖ كُلَّمَا دَخَلَ عَلَيْهَا زَكَرِيَّا الْمِحْرَابَ
وَجَدَ عِندَهَا رِزْقًا ۖ قَالَ يَا مَرْيَمُ أَنَّىٰ لَكِ هَٰذَا ۖ قَالَتْ هُوَ مِنْ عِندِ اللَّهِ ۖ إِنَّ اللَّهَ يَرْزُقُ مَن يَشَاءُ بِغَيْرِ حِسَابٍ

(So her Lord accepted her with good acceptance and caused her to grow in a good manner and put her in the care of Zechariah. Every time Zechariah entered upon her in the prayer chamber, he found with her provision. He said, “O Mary, from where is this [coming] to you?” She said, “It is from Allah. Indeed, Allah provides for whom He wills without account.”) (Ali-Imran: 37)

Since she was a child, this great bonoured child used to remember Allah (swt) in the prayer chamber, where she inspired the Prophet Zakaria (PBUH) a very important matter that Allah (swt) gives provision to whom He wills without account and without reasons:

 قَالَ رَبِّ إِنِّي وَهَنَ الْعَظْمُ مِنِّي وَاشْتَعَلَ الرَّأْسُ شَيْبًا وَلَمْ أَكُن بِدُعَائِكَ رَبِّ شَقِيًّا (4) وَإِنِّي خِفْتُ الْمَوَالِيَ مِن وَرَائِي وَكَانَتِ امْرَأَتِي عَاقِرًا فَهَبْ لِي مِن لَّدُنكَ وَلِيًّا (5) يَرِثُنِي وَيَرِثُ مِنْ آلِ يَعْقُوبَ ۖ وَاجْعَلْهُ رَبِّ رَضِيًّا

(He said, “My Lord, indeed my bones have weakened, and my head has filled with white, and never have I been in my supplication to You, my Lord, unhappy. (5) And indeed, I fear the successors after me, and my wife has been barren, so give me from Yourself an heir (6) Who will inherit me and inherit from the family of Jacob. And make him, my Lord, pleasing [to You].”) (Maryam: 4-6)

(… وَبِكُفْرِهِمْ ) (And [We cursed them] for their disbelief …): Because who says such things about Mary is indeed a disbeliever.