Tafsir of Al-Mutaffifeen 83:5

"For a Great Day," the magnitude of whose greatness cannot be appraised; for indeed, he who assumes this [accountability]—even if it be a weak assumption—would scarcely dare to commit such vile acts; so how then would one who is certain of it behave?

The day is described as "great" due to the greatness of what it contains, just as it is made the cause for the resurrection in consideration of what it holds. Some have posited an implied addition, meaning: "for the reckoning of a day." It has also been said that "assumption" (zann) here carries the meaning of "certainty," though the former interpretation is superior.

Al-Zamakhshari stated that Allah, glory be to Him, made them [the defrauders] in a worse state than the disbelievers. This is because He, Exalted is His Majesty, affirmed that the disbelievers possessed an "assumption" when He related their words: "We assume nothing but assumption" (45:32), whereas He, Mighty and Majestic is He, did not affirm it [the capacity for assumption] for them [the defrauders]. The intent is that Allah, Exalted be He, has placed them in the position of those who do not assume anything at all, so that the denunciation of their actions may be valid.