Muhammad said: It is read in two ways: ((ghulf)) and ((ghuluf)). The better of the two readings is ((ghulf)) with the sukun (vowel-less) on the lam, and its meaning is: having coverings (ghulf). The singular form of it is aghlaf. It is said: I put the sword in a sheath (ghilaaf), so it is an aghlaf sword. From this, it is said of one who has not been circumcised: aghlaf. It is as if they said: Our hearts are in vessels, similar to their saying: "Our hearts are within coverings (akinnah) from that to which you invite us."

And whoever reads it as ((ghuluf)), it is the plural of ghilaaf (vessel/cover). The meaning of this would be: Our hearts are vessels for knowledge, so why do they not understand what you say? [Verses 89-90]