1A ball used for games or recreation. Now chiefly North American: a ball used in a particular match or game, especially one presented after the game as a trophy to a player, coach, etc.
2Chiefly in racket sports: a position in a game in which one player or side requires a single point in order to win.
In predicative use: in very good health, spirits, etc.; fine, excellent.
Mid 19th century. From game + ball. Use as adjective in sense B. probably arose from use of the term in Gaelic handball.