What does « Ṣdif » mean in the Quran?

Say, "Have you considered: if Allah should take away your hearing and your sight and set a seal upon your hearts, which deity other than Allah could bring them [back] to you?" Look how we diversify the verses; then they [still] turn away.

Or lest you say, "If only the Scripture had been revealed to us, we would have been better guided than they." So there has [now] come to you a clear evidence from your Lord and a guidance and mercy. Then who is more unjust than one who denies the verses of Allah and turns away from them? We will recompense those who turn away from Our verses with the worst of punishment for their having turned away.

Bring me sheets of iron" - until, when he had leveled [them] between the two mountain walls, he said, "Blow [with bellows]," until when he had made it [like] fire, he said, "Bring me, that I may pour over it molten copper."