The term 'Diamond Man' refers to Jim, an umpire in the sport of baseball. Jim is a three-letter word and UMP is also a three-letter acronym for umpire. Jim is responsible for making sure that the rules of the game are followed and that the players are playing fairly. He also makes calls on close plays and keeps track of the score. Jim is an important part of the game and his presence ensures that the game is played fairly and safely.