Meaning: have a great time doing something enjoyable; a great deal to do, especially at someone else's expense.Origin: The phrase originates from the military. It was used in the literal sense, for a day spent in the field, doing maneuvers and drills, and the first reference is found in 1747, in Scheme Equip. Men of War. Over the years, it began to be used with reference to other events as well, and by the 20th century, the current meaning was well in use.