At the greatest distance away, or extending the most in space or time.
From Old English 'feor' meaning far, with superlative suffix '-est'; the 'th' developed through analogy with 'further' and 'farther' in Middle English.
English has both 'farthest' (distance) and 'furthest' (progress toward a goal), but we barely notice the distinction—yet it's a remnant of how words evolved differently!
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