They ask you to hasten in bringing chastisement1 upon them. Had there not been an appointed term for it, the chastisement would have already visited them; in fact it will come down upon them all of a sudden (at its appointed time) while they will not be aware of it.
Footnotes
[1]
Literally ‘knowing’.
[2]
That is, “They are challenging you again and again that If you are a true messenger and they are really denying the truth, then you should not delay the torment with which you threaten them.”