They are those who will suffer the worst chastisement in this life, and will be the greatest losers in the life to come.
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Tafseer
'Abdullāh Ibn 'Abbās / Muḥammad al-Fīrūzabādī
تفسير : (those) who have this attribute (are they for whom is the worst of punishment) a severe chastisement in the fire, (and in the hereafter) on the day of judgement (they will be the greatest losers) they will loose paradise and enter the fire.
Jalāl al-Dīn al-Maḥallī
تفسير : those are they for whom there is an awful chastisement, the worst [chastisement] in this world: being killed or being taken captive; and in the hereafter they will be the greatest losers, because they will end up in the fire which has been made everlasting for them.