Tafsir of An-Nazi'at 79:12

(They said) is a narration of another instance of their disbelief, which branches off from their previous disbelief. Perhaps the interposition of "they said" between them is to signal that the issuance of this disbelief from them is not by way of consistency and continuity, unlike their previous disbelief which was continuously issued from them at all times, as is indicated by its narration in the present tense form.

That is, they said by way of mockery, pointing to what they denied regarding the return to the "Hafira" (the former state), signaling its extreme remoteness from occurring: "That, then, would be a losing return." That is, a return possessing loss, or one whose possessors are losers—meaning, if that return were to be true, then we would be losers for having denied it. They presented what they were certain of its non-existence and impossibility in the form of something whose occurrence is highly probable, for the sake of greater mockery.

Al-Hasan said: "Khasirah" means "Kazibah" (a lie), meaning: that which does not exist. The meaning would thus be: "That, then, if we were decayed bones, would be a return that is not existent."