فَغَشّاها ما غَشّىٰ

Covering it with what covered it.

EXEGESIS

Ghashshā (covered) is from ghashy and ghashayān which literally means to cover with something. Ghishāwah is a blindfold, like in the verse, and put a blindfold (ghishāwah) on his sight (45:23).^{‎[1]Raghib, p. 607.} Here it means to be completely overtaken by the punishment of God, similar to the verse, they were engulfed (ghashiya) by what engulfed them of the sea (20:78).

EXPOSITION

In harmony with the earlier parts of the surah, especially verse 16 where a very similar phrasing is used, this verse is once again given the atmosphere of mystery with the usage of the expression with what covered it, without specifying the punishment. This also serves to underline the immensity and awe of the punishment, in a way suggesting it needs no description or is beyond description.^{‎[2]Tibyan, 9/439-440; Tabrisi, 9/277; Zamakhshari, 4/429.}

Many commentators have suggested that what covered it is referring to the stones that rained down from the sky on the people of Prophet Lot (a).^{‎[3]Tibyan, 9/439; Tabrisi, 9/277; Tabari, 27/47; Thalabi, 9/157.} Even if that is the case, the expression implies that it was so massive that it cannot be described.

[1] Raghib, p. 607.
[2] Tibyan, 9/439-440; Tabrisi, 9/277; Zamakhshari, 4/429.
[3] Tibyan, 9/439; Tabrisi, 9/277; Tabari, 27/47; Thalabi, 9/157.