Dishant Sunga

The use of lone "foo" as a nonsense word is pretty well documented in popular culture in the early 20th century, as is the military FUBAR. (Some background reading: FOLDOC FOLDOC Jargon File Jargon File Wikipedia RFC3092)

OK, so let's find some references.

STOP PRESS! After posting this answer, I discovered this perfect article about "foo" in the Friday 14th January 1938 edition of The Tech ("MIT's oldest and largest newspaper & the first newspaper published on the web"), Volume LVII. No. 57, Price Three Cents:

The Tech newspaper, a year earlier, the Letter to the Editor, September 1937:

And The Tech in December 1938:

The first documented "foo" in tech circles is probably 1959's Dictionary of the TMRC Language:

These are explained at FOLDOC. The dictionary's compiler Pete Samson said in 2005:

And from 1996's Jargon File 4.0.0:

A year before the TMRC dictionary, 1958's MIT Voo Doo Gazette ("Humor suplement of the MIT Deans' office") (PDF) mentions Foocom, in "The Laws of Murphy and Finagle" by John Banzhaf (an electrical engineering student):

Also 1964's MIT Voo Doo (PDF) references the TMRC usage:

But let's remember this question is about code examples, so let's find "foo", "bar" and "foobar" published in code.

So, Jargon File 4.4.7 says of "foobar":

The first published reference I can find is from February 1964, but written in June 1963, The Programming Language LISP: its Operation and Applications by Information International, Inc., with many authors, but including Timothy P. Hart and Michael Levin:

Also includes other metasyntactic variables such as: FOO CROCK GLITCH / POOT TOOR / ON YOU / SNAP CRACKLE POP / X Y Z

I expect this is much the same as this next reference of "foo" from MIT's Project MAC in January 1964's AIM-064, or LISP Exercises by Timothy P. Hart and Michael Levin:

It shares many other metasyntactic variables like: CHI / BOSTON NEW YORK / SPINACH BUTTER STEAK / FOO CROCK GLITCH / POOT TOOP / TOOT TOOT / ISTHISATRIVIALEXCERCISE / PLOOP FLOT TOP / SNAP CRACKLE POP / ONE TWO THREE / PLANE SUB THRESHER

For both "foo" and "bar" together, the earliest reference I could find is from MIT's Project MAC in June 1966's AIM-098, or PDP-6 LISP by none other than Peter Samson:

Some more recallations.

@Walter Mitty recalled on this site in 2008:

John V. Everett recalls in 1996:

Daniel P. B. Smith in 1998:

Robert Schuldenfrei in 1996:

Paul M. Wexelblat in 1992:

Bruce B. Reynolds in 1996:

Here's a straight IBM "BAR" from 1955.

Other early references:

I haven't been able to find any references to foo bar as "inverted foo signal" as suggested in RFC3092 and elsewhere.