The name foo fighter was coined by Allied aircraft pilots in World War II for mysterious aerial phenomena, such as glowing balls, seen in the skies over Germany. Originally used as a semi-derogatory reference to Japanese fighter pilots (known for erratic flying and extreme maneuvering), it became a catch phrase for fast moving, erratically flying objects (such as UFOs). It was thought at the time that they might be some German secret weapon, but the Germans were just as mystified by the phenomenon.