FOO is an abbreviation of Forward Observation Officer, a British Army
term in use as early as the First World War. The etymology of foo is
explored in the Internet Engineering Task Force (IETF) Request for
Comments 3092, which notes usage of foo in 1930s cartoons including
The Daffy Doc (with Daffy Duck) and comic strips, especially Smokey
Stover and Pogo. From there the term migrated into military slang,
where it merged with FUBAR.

source: wikipedia