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 Nazi secret weapon, but the Germans were just as mystified by the phenomenon.

During World War II, there was a cartoon character named Smokey Stover who used to say, "Where there's foo, there's fire." When U.S. pilots and sailors began seeing odd balls of light or shiny metal that could fly circles around our planes and which followed ships at sea, somebody called them Foo Fighters, and the name stuck. Others called them kraut fireballs because it was thought that they were some sort of German secret weapon.