At or to the greatest distance away, or to the greatest extent or degree.
From 'far' (Old English 'feor') with the superlative suffix '-est.' 'Further' is the comparative form, and 'furthest' is the superlative form (most far).
English speakers often mix up 'further' and 'farther,' but here's the tip: 'further' is usually about ideas and abstract concepts while 'farther' is physical distance—though 'furthest' works for both!
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