Explanation of the verse:

Nay, but their hearts were covered over by what (sins) they earned. [Al-Mutaffifīn: 14]

Al-Hasan (Al-Basrī) said, “It is because of doing one sin after another, until the heart becomes blind and dies.”

Qatādah also said, “It is because of doing one sin after another, one sin after another, until the heart dies and becomes black.”

Ibn Zayd said, “Their sins overcome their hearts until no good can get through to them.”

Mujāhid explained, “They used to consider the heart like a hand: when a person sins, his heart starts to scrunch up,” and he folded his little finger, “and when he does another sin,” he folded the next finger and continued until he had his fist clenched. “Then a seal is placed over it, and they used to say that this is the ‘covering’.”

Al-Tabarī in his Tafsīr, Sūrah Al-Mutaffifīn.