معتلث 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.

• مُعْتَلَثُ

  مُعْتَلَثُ الزِّنَادِ ذ [evidently, I think, a mistranscription, correctly ,] A man whose زِنَاد [pl. of زَنْدٌ] fail to produce fire. (L.) -b2- See also the following paragraph.…

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• مُعْتَلِثٌ

  مُعْتَلِثٌ ذ : see عَلِثٌ: -b2- and see the next preceding paragraph. -b3- Also, (O, * L, [thus in the latter, with kesr to the ل, but in the former without any vowel-sign to that letter, perhaps from اِعْتَلَثَ الزَّنْد…

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