There are many synonyms to describe a "smart fellow," each with their own unique connotation. Some words that come to mind include clever, intelligent, sharp, astute, quick-witted, savvy, bright, shrewd, knowledgeable, and resourceful. The term "brainy" can also be used to describe someone who is intellectually gifted. Additionally, the word "genius" is reserved for those with exceptional intelligence and ability. However, "genius" can also carry the implication of social awkwardness or eccentricity. Ultimately, the synonym chosen will depend on the context in which it is used and the specific qualities being emphasized.