Ruling on addressing someone as 'O you stupid'

Q: What is the ruling on addressing one's colleague saying, "O you stupid", "O you fool", or "O dull"? It is worthy mentioning that this may upset his colleague and leave him distraught. Is it permissible to say such words, even if the person they are said to does not mind? What is your advice to the youth who keep making fun of others?

A: It is not permissible for a Muslim to mock his Muslim brother or ascribe to him attributes that belittle him, or call him by names that he does not like, for Allah (Exalted be He) says, O you who believe! Let not a group scoff at another group, it may be that the latter are better than the former. Nor let (some) women scoff at other women, it may be that the latter are better than the former. Nor defame one another, nor insult one another by nicknames. How bad is it to insult one’s brother after having Faith [i.e. to call your Muslim brother (a faithful believer) as: "O sinner", or "O wicked"]. And whosoever does not repent, then such are indeed Zâlimûn (wrong-doers, etc.). The Prophet (peace be upon him) said, A Muslim is a brother to another Muslim. He should neither deceive him nor lie to him, nor leave him without assistance. Everything belonging to a Muslim is inviolable for a Muslim; his blood, his property, and honor. (Related by Ahmad and Al-Bukhari). Consequently, if it happens that a Muslim mocks his Muslim fellows or makes fun of them, he should hasten to repent to Allah and ask them to forgive him.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.