Tafsir of Al-Qalam 68:5

{The Afflicted (al-maftūn)}: The one possessed by madness, because he has been futina—that is, tested with madness.

Or, it is because the Arabs claim that madness is caused by the "confusion" (takbīl) of the Jinn, and they call the Jinn fattān (tempters/afflictors) for their destructive nature, with the bā’ being an extra particle.

Or, al-maftūn is a verbal noun (masdar) like al-ma‘qūl (reasoning) and al-majlūd (flogging). It means: "Which of you possesses the madness?" or "In which of the two groups among you does the madness reside: the group of the believers or the group of the disbelievers?"

That is: in which of them is found the one who deserves this title? This is an insinuation directed at Abū Jahl ibn Hishām, al-Walīd ibn al-Mughīrah, and their likes. This is similar to His saying, the Exalted: "They will know tomorrow who is the insolent liar" (Al-Qamar: 26).

{Indeed, your Lord is most knowing of who has strayed from His way, and He is most knowing of the [rightly] guided. So do not obey the deniers. They wish that you would compromise so they might compromise.}