اظلم doesn't appear in Hans Wehr's Modern Standard Arabic dictionary, but Lane's Arabic-English Lexicon has 2 entries for it, distinguished by diacritics. Select one below for the full classical Arabic definition.

• اظلم Form 4

  4 اظلم ذ , said of the night, (Fr, T, S, M, Msb, K,) and , (Fr, T, S, K,) the latter with kesr, (S,) like سَمِعَ, (K,) [erroneously written in the TT as from the M ظَلَمَ,] It became dark; (S, K;) or it became black; (M;…

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• اِظَّلَمَ Form 8

  8 اِظَّلَمَ ذ (T, S, M, K) and اِظْطَلَمَ and اِطَّلَمَ, (S, M,) which last is [said to be] the most usual, (S,) [but I have mostly found the first to be used,] of the measure اِفْتَعَلَ, (S, M,) He took upon himself [th…

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