It was recited: 'كسفا' with a pause and with movement, and both are the plural of 'كسفة', like 'قطع' and 'سدر'. It was said: 'الكسف' and 'الكسفة' are like 'الريع' and 'الريعة', and they mean 'the piece'. And 'كسفه' means 'he cut it'. And 'السماء' means 'the clouds' or 'the canopy'.

And their request for that was only due to their determination on denial and falsehood. If there had been in them the slightest inclination towards belief, they would not have even thought of it, let alone ask for it. The meaning is: If you are truthful that you are a prophet, then call upon Allah to send down upon us 'كسفا' from the sky.