This comes from a compound word meaning to speak well of something (our word “euphemism” comes from this Greek word). It refers to something that “deservedly enjoys a good reputation” (F. F. Bruce, New International Biblical Commentary, Philippians [Hendrickson], p. 146). As Paul says in 1 Corinthians 13, love believes the best about another person, it refuses to believe an evil report about a brother or sister until there is certain evidence to establish it.