1. interj. Term of disgust. 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, corge, grault, 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'), 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 in the on license plates of cars, in nonsens 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 may have been the Chinese word `fu' (sometimes transliterated `foo'), which can mean "happiness" or "prosperity" when spoken with the proper 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 resurfaced in popular American usage in 1995 via the name of one of the better grunge-rock bands). Because informants connected the term directly to the Smokey Stover strip, the folk etymology that connects it to French "feu" (fire) can be gently dismissed.
The U.S. and British militaries frequently swapped slang terms during the war (see kluge and kludge for another important example) Period sources reported that `FOO' became a semi-legendary subject of WWII British-army graffiti more or less equivalent to the American Kilroy. Where British troops went, the graffito "FOO was here" or something similar showed up. Several slang dictionaries aver that FOO probably came from Forward Observation Officer, but this (like the contemporaneous "FUBAR") was probably a backronym . Forty years later, Paul Dickson's excellent book "Words" (Dell, 1982, ISBN 0-440-52260-7) traced "Foo" to an unspecified British naval magazine in 1946, quoting as follows: "Mr. Foo is a mysterious Second World War product, gifted with bitter omniscience and sarcasm."
Earlier versions of this entry suggested the possibility that hacker usage actually sprang from "FOO, Lampoons and Parody", the title of a comic book first issued in September 1958, a joint project of Charles and Robert Crumb. Though Robert Crumb (then in his mid-teens) later became one of the most important and influential artists in underground comics, this venture was hardly a success; indeed, the brothers later burned most of the existing copies in disgust. The title FOO was featured in large letters on the front cover. However, very few copies of this comic actually circulated, and students of Crumb's `oeuvre' have established that this title was a reference to the earlier Smokey Stover comics. The Crumbs may also have been influenced by a short-lived Canadian parody magazine named `Foo' published in 1951-52.
An old-time member reports that in the 1959 "Dictionary of the TMRC Language", compiled at TMRC, there was an entry that went something like this:
FOO: The first syllable of the sacred chant phrase "FOO MANE PADME HUM." Our first obligation is to keep the foo counters turning.
(For more about the legendary foo counters, see TMRC.) This definition used Bill Holman's nonsense word, only then two decades old and demonstrably still live in popular culture and slang, to a ha ha only serious analogy with esoteric Tibetan Buddhism. Today's hackers would find it difficult to resist elaborating a joke like that, and it is not likely 1959's were any less susceptible. Almost the entire staff of what later became the MIT AI Lab was involved with TMRC, and the word spread from there.
Finally (and perhaps irrelevantly) a Russian correspondent reports that in mainstream Russian, "Foo" (or "Fu") is an interjection commonly used as a response to bad smell, bad taste, or other unpleasant sensatiion.