Wiktionary
n. Any of various unidentified flying objects, as reported by Allied pilots during World War II.
Wikipedia
The term foo fighter was used by Allied aircraft pilots in World War II to describe various UFOs or mysterious aerial phenomena seen in the skies over both the European and Pacific theaters of operations.
Though "foo fighter" initially described a type of UFO reported and named by the U.S. 415th Night Fighter Squadron, the term was also commonly used to mean any UFO sighting from that period. Formally reported from November 1944 onwards, witnesses often assumed that the foo fighters were secret weapons employed by the enemy.
The Robertson Panel explored possible explanations, for instance that they were electrostatic phenomena similar to St. Elmo's fire, electromagnetic phenomena, or simply reflections of light from ice crystals.
Usage examples of "foo fighter".
General Gullick cut in, sur- power on board Bouncer Three in conjunction with a near prising everyone in the room with the harshness of his collision with the foo fighter.