TO kill a mocking tongue

Al-Ma'roor ibn Suwayd narrates that he once saw Abu Dharr radi Allahu anhu wearing a beautiful shawl. His slave standing next to him was wearing a shawl exactly like it, warm and beautiful. Ma'roor said to Abu Dharr, “Perhaps you could take the shawl of your servant and give him another (less expensive) one.”

“Never,” said Abu Dharr, “for I once had a servant whose mother was not Arab and I cussed him and his mother. That servant went to the Messenger of Allah sal Allaahu alayhi wa sallam complaining of the words I had said. When RasulAllah sal Allaahu alayhi wa sallam saw me he commented, ‘O Abu Dharr, you are a man who still has jahiliyyah (pre-Islamic ignorance) in him.’”

Because of these painful words, Abu Dharr radi Allahu anhu would always dress his servants in the exact same garments that he would wear.
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Perhaps the one that is being made fun of is more beloved to Allah. Subhan Allah! Let us remember this if we ever try to make fun of someone, perhaps Allah loves them and does not love us. Didn't the mushrikeen make fun of RasulAllah sal Allaahu alayhi wa sallam, and we know Allah loved him and not them. Didn't the munaafiqeen make fun of the Sahaabaa, and we know Allah loved the Sahaabaa and not them.

RasulAllah sal Allaahu alayhi wa sallam said:

“Verily, a person will speak words from those that Allah hates, paying no heed to what he is saying, and with those words he will plummet in to Hellfire” (Bukhari).

Allah ta'ala shows us in Surah Al-Mutaffifeen how this characteristic of laughing at others is a characteristic of the kuffar:

Indeed, those who committed crimes used to laugh at those who believed (Al-Mutaffifin 83/29).

The seriousness of this sin varies in accordance to the subject being insulted. On the highest level of seriousness is to make fun of Allah, His aayaat, or His Messenger sal Allaahu alayhi wa sallam.