As adjectives the difference between furthest and furtherest
is that furthest is superlative of far while furtherest is furthest.
As an adverb furthest
is superlative of far POS=adverb.
furthest
English
Adjective
(head)
(far)
# Most distant in time, space or degree.
Adverb
(head)
# To the greatest degree or extent, or to the most distant point in time or space.
Synonyms
* farthest
furtherest
English
Adjective
(head)
(nonstandard, humorous use only, informal) furthest