وَجَعَلْنَا مِنۢ بَيْنِ أَيْدِيهِمْ سَدًّا وَمِنْ خَلْفِهِمْ سَدًّا فَأَغْشَيْنَـٰهُمْ فَهُمْ لَا يُبْصِرُونَ Qur’an Yasin (36:9)WajaAAaln a min bayni aydeehim saddan wamin khalfihim saddan faaghshayn a hum fahum l a yub s iroon a
Sc., "so that they can neither advance nor go back": a metaphor of utter spiritual stagnation.
Their retreat is cut off and their progress is impossible. Further the Light that should come from above is cut off, so that they become totally devoid of any hope, and the last gleam of any spiritual understanding is extinguished in them.