Tafsir of Sad 38:57

"This" is a predicate for a deleted subject, meaning "This is the punishment." His saying, the Exalted, "So let them taste it" is a sentence sequential to the one preceding it, functioning like the result of a deleted conditional.

His saying, "Hamim (scalding water) and Ghassaq (foul discharge)," is a predicate for a deleted subject, meaning "It is Hamim and Ghassaq," where "this" may be used to point to multiple things. Alternatively, it is a subject whose predicate is deleted, meaning "Of it is Hamim, and of it is Ghassaq," as in the verse: "Until when the morning shines in darkness (ghasaq), and the herbs are left twisted and harvested," meaning "Of them are twisted, and of them are harvested."

Alternatively, "This" is a subject whose predicate is "Hamim," and the sentence "so let them taste it" is a parenthetical clause, similar to your saying "Zaid—so understand—is a righteous man." Or, "This" is a subject whose predicate is "so let them taste it," following the position of al-Akhfash in his allowance of [the construction] "Zaid—so strike him," citing as evidence the saying: "And a woman of Khawlan said: So marry their young women."

Or "This" is in the accusative case due to an implied verb explained by "so let them taste it," meaning "Let them taste this, so let them taste it." Perhaps you might prefer the view that "This" is a subject and "Hamim" is its predicate, and what lies between them is a parenthetical clause; this is the view preferred in al-Kashshaf. The "fa" is explanatory and sequential, indicating that they will have a tasting after a tasting. Regarding "Hamim" and "Ghassaq" in these two aspects, the two possibilities mentioned previously remain.

Hamim is water of extreme heat.

Ghassaq—with the shaddah (doubling of the letter), as read by Ibn Abi Ishaq, Qatadah, Ibn Wathab, Talhah, Hamzah, al-Kisa'i, Hafs, al-Fadl, Ibn Sa'dan, and Harun from Abu 'Amr; and with the light (non-doubled) form, as read by the rest of the seven—is a name for that which flows from the pus of the people of the Fire, as narrated from 'Ata, Qatadah, and Ibn Zaid. From al-Suddi, it is what flows from their tears. Ibn Jarir extracted from Ka'b that it is a spring in Hell into which the venom of every venomous creature—snake, scorpion, and others—flows; the disbeliever is dipped into it, causing his skin and flesh to fall off. Ibn Jarir and Ibn al-Mundhir extracted from Ibn Abbas that it is the intense cold (zamharir). It is said: It is, whether doubled or light, an adjective derived from ghasaqa—like daraba and sami'a—meaning "it flowed." It is said, "The eye ghasaqat," meaning its tears flowed. Thus, based on what is in al-Bahr, it is an adjective whose noun has been omitted, meaning "and a tasted ghassaq," intended to mean a fluid from the skins of the people of the Fire, for example. The adjectival nature is more evident in the doubled form, because fa''al with a shaddah is rare in nouns, though examples include ghayyad (a name for the owl), khattar (an oil made from olives), and 'aqar (plants used for medication). Among the strange assertions is that of al-Jawaliqi and al-Wasiti, who claimed that ghassaq is the "cold and foul-smelling" in the Turkish language. The truth is that it is Arabic; yes, foulness is an attribute of it in reality, but it is not included in the definition. Ahmad, al-Tirmidhi, Ibn Hibban, and a group—authenticated by al-Hakim—extracted from Abu Sa'id that the Messenger of Allah, peace be upon him, said: "If a bucket of ghassaq were poured into the world, it would cause all the people of the world to stink." It is also said that ghassaq is a punishment known only to Allah, the Mighty and Majestic, though this report makes that unlikely.