Tafsir of Al-Mu'minoon 23:66

"My verses were indeed recited to you..." The saying of the Exalted is explicit that this is the justification for the non-arrival of help from His side, due to their disbelief in the verses. If the denied help were imagined to be from others, it would have been justified by their inability or by the might and power of Allah, the Exalted. You know that they are the polytheists whose partners were set before their eyes, and the act of wailing was not restricted to being directed toward Allah, the Exalted. The matter of the justification is straightforward. It may be said that the meaning, in this respect, is the claim of the one crying for help, for it neither prevents us from harming you nor does it benefit you with us; you have committed a grave matter and a great sin that this [wailing] does not avert. Furthermore, what is in the words of the objector is clearly far-fetched. The intended meaning is: "My verses were indeed recited to you" before the punishment seized your affluent ones, "so you were" upon their recitation "turning back on your heels," meaning you were turning away from hearing them, let alone believing in them and acting upon them.

Nukus (turning back) is returning. A'qab (heels) is the plural of aqib, which is the back of the foot. A person returning on his heels is his returning on the path he first took, as it is said, "He returned his return to his beginning." Some have held that the restriction to the "heels" is for the sake of emphasis, as in "I saw him with my own eyes," based on the view that nukus is retreating backward, and [that the addition of] "on the heels" [is to clarify the manner]. In any case, it is a metaphor for turning away.

Ali, may Allah, the Exalted, ennoble his countenance, read tankusun (turning back) with a damma on the kaf.