In the fifth place or as the fifth point in a sequence or argument.
From fifth (ordinal form of five) plus -ly (adverb suffix). This ordinal adverb is used for listing and organizing ideas, dating back to Middle English.
Academic writers use 'fifthly' when they've already used 'firstly,' 'secondly,' 'thirdly,' and 'fourthly'—but most modern writing avoids this style because it sounds stuffy and old-fashioned.
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