In the eighteenth position or order; used to introduce the eighteenth item in a list or sequence.
From 'eighteenth' (eighteenth in order or position) plus the adverbial suffix '-ly'. The '-th' ending comes from Old English ordinal formation.
While we often say 'finally' instead of 'eighteenthly,' medieval scholars loved listing things with such ordinal adverbs—'firstly,' 'secondly'... all the way through alphabetical arguments!
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