In the fourteenth place or position in a sequence; as the fourteenth point in a list.
From 'fourteenth' (ordinal form of 'fourteen') plus the adverbial suffix '-ly' (from Old English '-lice'). Rarely used but constructed following standard English word-formation rules.
While technically valid, 'fourteenthly' is so rare that most writers avoid it—nobody wants to sound like they're reading a tedious legal document with 14+ points to cover!
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