حِجْرًا مَّحْجُورًا (25:22) The literal meaning of حِجر is protection, and مَّحْجُورًا is fully protected. This word was used in Arabic language at the time of danger, when there was a danger in sight. The word was used to seek protection - that is to ask for protection against the imminent danger. Likewise, on the Dooms Day when the infidels will see angels approaching toward them with chastisement, they will utter this word as per their habit in this world to seek protection. Sayyidna Ibn ` Abbas ؓ has described the meaning of this word حَرَاماً مُحَرَّماً (that is banned, prohibited).
According to this interpretation, the phrase will mean that on the Dooms Day when the infidels will see the angels approaching them with chastisement, they will seek their pardon and plead to let them in the paradise. In response to their pleading the angels will reply حِجْرًا مَّحْجُورًا that is the Paradise is banned and prohibited for infidels. (Mazhari)