What Is the Origin of the Saying "Once the Balloon Has Gone Up"?
Once the balloon has gone up means when trouble is here or in a period of trouble.
The term once the balloon has gone up derives from the First World War. Whenever enemy activity was expected, observations balloons would be released to monitor the enemy troop movements. As a result, the raising of these balloons, which were visible to all, soon became a sign of pending enemy action.
A Competing Theory
The term when the balloon goes up refers to the large barrage balloons which were raised on steel cables above British cities during the Second World War. The idea was that enemy bombers would keep away from the cities, fearing that the steel cables would slice through their wings. Therefore, when the barrage balloons went up, it was a sign for a city's inhabitants of a pending air raid.
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