وَمَا هُوَ بِٱلْهَزْلِ
The preceding verse gave an affirmative ruling (it is a word that separates); this verse gives a negative one (it is no joke).
This is another example of the method treated in Sūrat al-Ikhlāṣ: in describing a thing, to say both what it is and what it is not. The relation of negation (nafy) and affirmation (ithbāt) set up between al-Kāfirūn and al-Ikhlāṣ is here gathered into a single pair of verses.
Wa mā huwa bi'l-hazl — the bāʾ that enters the predicate of a negative sentence adds nothing to the meaning but strengthens the negation. In Arabic this is one of the most forceful forms of negation.
The same use is widespread in the Qurʾān: "God is not one to wrong His servants" (bi-ẓallāmin, at Āl ʿImrān 3/182).
Further, al-hazl has come definite (with the article). Had it come indefinite ("it is not a joke") the meaning would have been close; its coming definite rejects the whole genus: this is not a thing of the sort called a joke.
Root: ه-ز-ل. The word has two wings of meaning, and the lexicographers give them under the same entry:
1. The opposite of seriousness; joke, jest, idle talk. In Arabic jidd (seriousness) and hazl are a classical pair of opposites. Hāzil — one who jests.
2. To grow thin, to become lean, to waste away. Hazala — he grew thin; huzāl — thinness, leanness; hazīl — scrawny.
The lexicographers connect the two senses thus: a joke is speech that has been emptied out. Serious speech is full, it carries a load, it produces a result; a joke is speech in a weakened, lightened state, with its binding force taken away. In English too the expressions "empty talk" and "a light remark" carry the same intuition.
This connection is of the kind the lexicographers set up, and it cannot be presented as a settled etymological verdict. But it is worth recording, because it sharpens what the verse says: "This word is not weak."
The Qurʾān regularly records the charges brought against it: poetry, sorcery, soothsaying, tales of the ancients, fabrication. Their common denominator is this: all of them aim at lowering the seriousness of the text. The cheapest way of refuting a word is to declare it not worth arguing about.
And this is the same logic as the observation recorded in the section on Sūrat al-Ḍuḥā: there too it was said that the source was cut off without entering into the content of the claim. Here too the weight of the word is emptied out without entering into the content.
The verse rejects this move. And in rejecting it, it does not enter into argument — it says "it is this, it is not that". Because whether a word is serious is found out not by argument but by its consequences.
The first ten verses gave a report: you are being watched, you will be brought back, what is inside you will be probed. Now the text stops and speaks about itself: what I have said is serious.
Then the last three verses will say what is to be done about those who do not accept this seriousness.