In the fourth place or as the fourth point in a sequence or argument.
From 'fourth' (Old English 'feortha') plus the adverbial suffix '-ly' (from Old English '-lice'). Follows the pattern of 'thirdly' and 'firstly'.
Academic writers love 'firstly, secondly, thirdly, fourthly' but linguists have noted that 'fourth' originally had no '-ly' form—it crept in by analogy, creating this oddly formal sound.
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