What is the ruling on public slander with the intention of insult, in cases where societal custom dictates the use of slanderous words for insult?

A Muslim is commanded to guard his tongue, for every word a servant utters is recorded against him. A person may utter a word that casts him into the Hellfire, further than the distance between the East and the West. Slandering a Muslim is one of the major sins, and likewise, insulting him is forbidden, for the Prophet, peace and blessings be upon him, said: "Insulting a Muslim is فسق (disobedience), and fighting him is كفر (disbelief)." The legal difference between them is that slander (القذف) is accusing a person of adultery or denying his lineage, and its prescribed punishment is eighty lashes. As for insult (السب), it is reviling someone with words other than that, and it does not have a prescribed punishment, but rather a discretionary one (التعزير). The fact that listeners know the accuser is lying does not affect the ruling of slander, nor does common societal practice permit it. However, if the expression is ambiguous and not explicit slander, then reference is made to custom: if people consider it slander, then it is slander.