Is saying the insult "You are the son of a fornicator" to someone, even unintentionally, considered an accusation of unchastity against chaste women (Qadhf al-muhsinat)?

If you utter the ugly word of slander unintentionally, and your tongue slips, this is forgiven between you and Allah. However, if you intentionally utter it without intending the meaning of slander, this is not an excuse. In the Hadith (definition): "Indeed, a servant may utter a word that displeases Allah, paying no heed to it, and he will fall into Hellfire because of it." Slandering chaste women is one of the greatest sins, and the slanderer must be punished if the slandered person or their guardian demands it. If the slandered mother is deceased, the slanderer is to be punished if the free Muslim son demands it.

You must offer sincere repentance for this sin by refraining from it, regretting it, and resolving not to return to it, along with seeking forgiveness from the wronged party. If your friend's mother is alive and aware of the slander, you must seek her forgiveness. However, if she is deceased, or alive but unaware of your slander against her, it is sufficient for you to deny yourself in front of your friend; because the repentance of the slanderer is by denying oneself.