The word (عَمَّ ) is originally (عَنْ مَا ), which means ‘About what’.
مَا : It is used to emphasize the obscure. This is due to the importance and greatness of this question.
The question is: About what are the disbelievers of Makkah asking one another?
There is a note of condemnation in this question, as human beings ought not doubt the Day of Accountability. However, since they do doubt it, there are questions, discussions and objections, exactly as what happens nowadays since we can find those who are doubtful regarding the Qur’an and Allah (swt) in every period of time.