furthest (adj., adv.)

late 14c., formed as superlatives to further (adj. and adv.).

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Definitions of furthest from WordNet
furthest (adv.)
to the greatest degree or extent or most advanced stage (`furthest' is used more often than `farthest' in this abstract sense);
went the furthest of all the children in her education
furthest removed from reality
Synonyms: farthest
furthest (adv.)
to the greatest distance in space or time (`farthest' is used more often than `furthest' in this physical sense);
he swam the furthest
Synonyms: farthest
furthest (adj.)
(comparatives of `far') most remote in space or time or order;
explored the furthest reaches of space
Synonyms: farthermost / farthest / furthermost / utmost / uttermost