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.
The WW II-era term for a UFO, that U.S. pilots and intelligence believed to be experimental/advanced Nazi aircraft. But may have been actual UFO's.
Also interesting that it's comics-related, based on a "foo" catch-phrase from a 1930's comic strip that one of the pilots was an avid reader of.
There's a whole myth/conspiracy theory, expanding from that belief in Nazi-made UFO aircraft, that expands into secret weapons, and secret Nazi colonies in Antarctica, and that Hitler and other Nazis might have survived the war, living in a secret utopia, or even expanding to space travel, time travel, or planning a new war of global conquest. Or that the Nazis obtained their advanced technology from a crashed alien ship. A Nazi Roswell.