1. Any unidentified flying object described as a ball of fire or light
So, that band didn't come up with this one on their own?
Foo fighter is first attested in 1945, though it's likely a bit older since it's US military slang. It comes from a nonsense word created by American cartoonist Bill Holman in his strip 'Smokey Stover.'
Here's a comic from the 1930's which includes the word foo: