Definitions of foo  

2. [very common] Used very generally as a sample name for absolutely anything, esp. programs and files (esp. scratch files).

3. First on the standard list of metasyntactic variables used in syntax examples. See also bar, baz, qux, quux, garply, waldo, fred, plugh, xyzzy, thud.

When ‘foo’ is used in connection with ‘bar’ it has generally traced to the WWII-era Army slang acronym FUBAR (‘Fucked Up Beyond All Repair’ or ‘Fucked Up Beyond All Recognition’), later modified to foobar. Early versions of the Jargon File interpreted this change as a post-war bowdlerization, but it it now seems more likely that FUBAR was itself a derivative of ‘foo’ perhaps influenced by German furchtbar (terrible) — ‘foobar’ may actually have been the original form.

For, it seems, the word ‘foo’ itself had an immediate prewar history in comic strips and cartoons. The earliest documented uses were in the Smokey Stover comic strip published from about 1930 to about 1952. Bill Holman, the author of the strip, filled it with odd jokes and personal contrivances, including other nonsense phrases such as “Notary Sojac” and “1506 nix nix”. The word “foo” frequently appeared on license plates of cars, in nonsense sayings in the background of some frames (such as “He who foos last foos best” or “Many smoke but foo men chew”), and Holman had Smokey say “Where there's foo, there's fire”.

According to the Warner Brothers Cartoon Companion Holman claimed to have found the word “foo” on the bottom of a Chinese figurine. This is plausible; Chinese statuettes often have apotropaic inscriptions, and this one was almost certainly the Mandarin Chinese word fu (sometimes transliterated foo), which can mean “happiness” or “prosperity” when spoken with the rising tone (the lion-dog guardians flanking the steps of many Chinese restaurants are properly called “fu dogs”). English speakers' reception of Holman's ‘foo’ nonsense word was undoubtedly influenced by Yiddish ‘feh’ and English ‘fooey’ and ‘fool’.

Holman's strip featured a firetruck called the Foomobile that rode on two wheels. The comic strip was tremendously popular in the late 1930s, and legend has it that a manufacturer in Indiana even produced an operable version of Holman's Foomobile. According to the Encyclopedia of American Comics, ‘Foo’ fever swept the U.S., finding its way into popular songs and generating over 500 ‘Foo Clubs.’ The fad left ‘foo’ references embedded in popular culture (including a couple of appearances in Warner Brothers cartoons of 1938-39; notably in Robert Clampett's “Daffy Doc” of 1938, in which a very early version of Daffy Duck holds up a sign saying “SILENCE IS FOO!”) When the fad faded, the origin of “foo” was forgotten.

One place “foo” is known to have remained live is in the U.S. military during the WWII years. In 1944-45, the term ‘foo fighters’ was in use by radar operators for the kind of mysterious or spurious trace that would later be called a UFO (the older term resurface