In the eighth position or order in a sequence; used as the eighth item in an enumerated list.
From 'eighth' (ordinal form of 'eight') plus the adverbial suffix '-ly'. Part of the ordinal adverb system in English.
Medieval scholars were obsessed with listing things—'firstly, secondly, thirdly...' all the way to 'eighthly'—it was a way to show intellectual organization before modern outline formats!
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