Netherworld, Underworld, Chthonian World) is in Islamic belief either a prison, vehement torment or straitened circumstances at the bottom of Jahannam or hell, below the earth (compare Greek Tartarus),[1][2]: 166 or, according to a different interpretation, a register for the damned or record of the wicked,[3] which is mentioned in Quran 83:7.
The word as an adjective means "vehement" or "intense" and is derived from the root S-J-N (س ج ن) related to gaoling or imprisonment.
[4] A similar-sounding word (but of unrelated etymology from Byzantine Greek σιγίλλιον sigíllion via Classical Syriac), sijill (Arabic: سِّجِلّ) appears in a verse (21:104) and is translated as "scroll".
[2]: 187 In his work The Incoherence of the Philosophers, he explains that this doesn't lower the deceased experience of the afterlife, but they perceive pleasure and pain like when they were alive.
[2]: 187 According to The Alchemy of Happiness, sijjin will be a manifestation of the earthly life, presented to those who pursued worldy matters instead of religious bliss.