When some activity or enterprise begins. This expression dates from World War I, when the British artillery would send up a balloon to signal gunners along the line to begin firing. In time the term was extended to any undertaking, such as the opening of a store, the time a court session begins, and so on. John Braine used it in his novel Room at the Top (1957): “Merely because I let you give me a beery kiss in the Props Room, you think the balloon’s going up.” The term may be dying out, however, at least on this side of the Atlantic.