piss call (noun)1. In boarding schools and camps for young boys, a call made in the middle of the night to go to the urinals. This helped prevent bed-wetting.
2. In the armed services, the same type of call jokingly made for any alarm in the middle of the night, as these were usually scary and might cause bed-wetting.
Hence, Piss-Call Charlie, a World War II term for enemy planes that made night passes. They usually just dropped a bomb or two, but that was enough to wake everyone up and frighten them.