Question: Is there any sin for which the doer’s repentance will not be accepted?
Answer: The sound view is that there is no sin whose perpetrator is denied repentance, for Allah—glorified and exalted—accepts the repentance of a disbeliever if he turns back. The Most High says, “Say to those who disbelieve: If they desist, what has already passed will be forgiven them; but if they return, then the precedent of the former peoples has already occurred” (8:38). Likewise, should a polytheist repent—even though associationism is the gravest of sins—Allah will accept his repentance. Thus, according to the sounder of the two scholarly opinions, there is no sin from which repentance is not accepted.
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Exegesis of Sura al-Nisāʾ

Question: What is the meaning of His statement, “Allah does not like the public mention of evil words except by one who has been wronged; and Allah is All-Hearing, All-Knowing” (4:148)?
Answer: One interpretation is that no one may call down curses upon another unless the person cursed has wronged him; in that case the wronged party may supplicate against the one who oppressed him. Another interpretation is that if someone insults you, you may respond in kind by way of lawful retaliation, whereas cursing and insulting without cause is impermissible. Thus, “Allah does not like the public mention of evil words except by one who has been wronged”: if someone abuses or reviles you, you may retaliate with the like, but if there is no provocation, raising one’s voice with abuse is not allowed, for that is uttering evil openly.

Question: Then the “evil” (sūʾ) referred to here means cursing or verbal abuse?
Answer: Yes—cursing, reviling, or supplicating against someone.