Foo was a nonsense word used by cartoonist Bill Holman in the comic strip "Smokey Stover" from the 30's into the 50's. He was reported to have discovered the word in a Chinese fortune cookie and the word appeared in almost every panel in the strip. It predates most of definitions that I have seen so far. The strip appeared in the SF Chronicle or the SF Examiner.
Foo was on signs along the road, on license plates or anywhere else one could imagine