Tafsir of Al-Fajr 89:13

"Whip" (sawt) is originally an infinitive from the verb sata-yasutu, meaning "to mix." A poet once said: "O Harith, if our blood were to be mixed (tuyat), they would separate so that no blood would touch another." It became commonly used for the braided strap used for striking, and it was so named because its strands are intertwined, or because it mixes flesh with blood.

The expression of its descent as "pouring" (sabb) is to signify its abundance, continuity, and persistence; for "pouring" describes the spilling of a liquid—or that which follows its path in flowing, such as grains or sand—by emptying it with intensity, abundance, and continuity. Attributing this to a "whip"—even though, as you have heard, it is not of that nature—is a metaphor for the speed of its descent, similar to a poured substance.

It is said that naming what was sent down a "whip" is to signal that it is, in its magnitude, small relative to what is prepared for them in the Hereafter, just as a whip is small compared to other instruments of torture. In al-Kashf, it is stated that the genitive construction of "whip" to "punishment" diminishes the scale of what befell them, and this does not contradict the expression "pouring," which indicates abundance, for "little" and "much" are relative matters.

It is also permitted that "punishment" (al-adhab) here refers to the act of tormenting (at-tadhib), in which case the genitive construction signifies "for" (lam), while the expression of "pouring" and the naming of the "whip" remain as previously stated. The verse is in the same category as the Almighty’s saying: "So God made them taste the garment of hunger."

It is also possible that the genitive construction is like the one in "a silver [expanse] of water," meaning: "Your Lord poured upon them a punishment like a whip," in the sense of various types of punishment mixed together like the intertwining of the whip’s strands. Or, that "whip" is an infinitive used in the sense of the passive participle, with the construction being like "a [well-worn] garment of velvet," meaning: "Your Lord poured upon them a punished [i.e., mixed] punishment," implying that He poured out types of punishment, each mixed with the other. In al-Sihah, "a whip of punishment" is interpreted as "a share of punishment," or it may refer to its intensity, for punishment may be inflicted by a whip. The objective is to provide a vivid illustration, and what is most appropriate for the excellence of the Revelation is what has preceded.