Abarim Publications' online Biblical Greek Dictionary
The familiar adverb ευ (eu) means good or noble. It occurs a mere 6 times independently in the New Testament, see full concordance, used in constructions like "that it may be well with you" (Ephesians 6:3), or "well done!" (Luke 19:17).
But mostly our word is used as a prefix and is as such part of an extensive array of nouns and verbs, sometimes to distinguish a regular item from an outstanding version of it, but mostly as an intensitive.
Our word survives in English in words like Eucharist and Euchologion (an Orthodox book of prayer), or even euphoria, euphemism, eucalyptus, and others.