The adverbial form of megas (great). Means greatly, exceedingly, to a great degree. Used to express the intensity of spiritual experiences and responses.
Paul told the Philippians that he rejoiced 'greatly in the Lord' when they renewed their concern for him (Philippians 4:10). This word modifies and intensifies — it describes the degree of rejoicing, sorrow, or any other experience. Paul's great joy came not from comfortable circumstances (he was in prison) but from the Lord. The greatest joys are spiritual, not material.