Relating to the Essenes; of or pertaining to Essenism or the practices and beliefs of this Jewish sect.
From Essene + -ical (combining -ic and -al, both meaning 'of or relating to'). The doubled suffix reflects older scholarly English usage.
Medieval and Renaissance scholars loved adding -ical to everything—it made words sound more authoritative and scholarly, which is why we still see it in academic writing today!
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