<jargon> /foo/ A sample name for absolutely anything, especially programs and files (especially scratch files). 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.
However, the use of the word "foo" itself has more complicated antecedents, including a long history in comic strips and cartoons.
"FOO" often appeared in the "Smokey Stover" comic strip by Bill Holman. This surrealist strip about a fireman appeared in various American comics including "Everybody's" between about 1930 and 1952. FOO was often included on licence plates of cars and in nonsense sayings in the background of some frames such as "He who foos last foos best" or "Many smoke but foo men chew".
Allegedly, "FOO" and "BAR" also occurred in Walt Kelly's "Pogo" strips. In the 1938 cartoon "The Daffy Doc", a very early version of Daffy Duck holds up a sign saying "SILENCE IS FOO!". Oddly, this seems to refer to some approving or positive affirmative use of foo. It has been suggested that this might be related to the Chinese word "fu" (sometimes transliterated "foo"), which can mean "happiness" when spoken with the proper tone (the lion-dog guardians flanking the steps of many Chinese restaurants are properly called "fu dogs").
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.
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.
Another correspondant cites the nautical construction "foo-foo" (or "poo-poo"), used to refer to something effeminate or some technical thing whose name has been forgotten, e.g. "foo-foo box", "foo-foo valve". This was common on ships by the early nineteenth century.
Very probably, hackish "foo" had no single origin and derives through all these channels from Yiddish "feh" and/or English "fooey".