In the third place or as the third point in a sequence of items or arguments.
From third (Old English 'þridda,' from three) + -ly (adverb suffix). It's been used in formal writing and speech since at least the 1600s.
Ordinal adverbs like 'thirdly' are fascinating because they're disappearing from modern writing—linguists have noticed 'thirdly' and 'fourthly' are becoming rare as people prefer 'third' or just 'next' in casual speech!
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