In a manner that furthers or helps progress something forward; additionally or at a greater distance.
From 'further' plus the adverbial suffix '-ly.' This is a logical but rarely used formation in modern English, appearing mostly in archaic or dialectal texts.
This adverb almost never appears in modern writing because we use 'furthermore' or 'further' instead—but it shows how English speakers logically add '-ly' to any adjective to make an adverb, even if some combinations never catch on.
Complete word intelligence in one call. Free tier — 50 lookups/day.