Farther, farthest or further, furthest?
Farther and further are comparative adverbs or adjectives. They are the irregular comparative forms of far. We use them to talk about distance. There is no difference in meaning between them. Further is more common: …
furthest
adjective
uk /ˈfɜː.ðɪst/ us /ˈfɝː.ðɪst/
superlative offar: （far的最高級）
The novel explores the furthest extremes of human experience.
The accomplishment marks the furthest point that scientists have yet reached in cloning a nonhuman primate.
Peace was the furthest thing from the administration's agenda.
At its furthest reaches the city gradually gives way to trees.
The machair is the rich blanket of wild plant and animal life unique to the furthest fringes of Scotland and Ireland.

SMART Vocabulary: related words and phrases

Distant in space and time
(in) the middle of nowhereidiom
afar
afield
all/the four corners of the world/earthidiom
anywhere
as far as the eye can/could seeidiom
away
far
from afar
further
furthermost
light years awayidiom
lonely
middle
remotely
remoteness
ultra
ultra-distant
virtually
wide

furthest | American Dictionary

furthest
adjective, adverb
us/ˈfɜr·ðəst/

furthestadjective, adverb (DISTANCE)

superlative offar :
The furthest west I’ve ever gotten is the Grand Canyon.

Note:

Both furthest and farthest are used as adverbs and adjectives. Farthest is more commonly used when referring to distances. Furthest is more often used when the intended meaning is figurative.

furthestadjective, adverb (AMOUNT)

superlative offar :
At that time, the countries furthest along in terms of economic development were Chile, Mexico, and Venezuela.
furthest
pronoun
us/ˈfɜr·ðəst/

furthestpronoun (GREATEST AMOUNT)

the greatest amount:
This is the furthest our soccer team has advanced in the playoffs.