Used to describe physical spaces, usually vast and challenging to reach. The lighthouse was on the furthermost cliff of the coastline.
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Measurement Irrelevant
This term applies even if the exact distance is unknown; it implies an extremity. From all the districts, hers was the furthermost from the city center.
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Metaphorical Use
Can be used figuratively to stress on the effort needed to achieve something. He went to the furthermost reaches of his mind to recall the ancient lore.