An archaic or variant spelling of 'furthest,' meaning the most distant or most advanced degree.
From Middle English, representing an older superlative form of 'far' before standardized spelling. Likely influenced by the comparative 'further,' this variant fell out of use by modern English.
This spelling shows how English was much more flexible with spelling before dictionaries standardized everything—people wrote what sounded right to them, and 'furtherest' made logical sense even if we don't use it anymore.
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