The Free On-line Dictionary of Computing (19 January 2023):

foobar

    Another common metasyntactic variable; see foo.
   Hackers do *not* generally use this to mean FUBAR in either
   the slang or jargon sense.

   According to a german correspondent, the term was coined
   during WW2 by allied troops who could not pronounce the german
   word "furchtbar" (horrible, terrible, awful).

   [Jargon File]

   (2003-07-03)


The Jargon File (version 4.4.7, 29 Dec 2003):

foobar
 n.

    [very common] Another widely used metasyntactic variable; see foo for
    etymology. Probably originally propagated through DECsystem manuals by
    Digital Equipment Corporation (DEC) in 1960s and early 1970s; confirmed
    sightings there go back to 1972. Hackers do not generally use this to mean
    FUBAR in either the slang or jargon sense. See also Fred Foobar. In
    RFC1639, “FOOBAR” was made an abbreviation for “FTP Operation Over Big
    Address Records”, but this was an obvious backronym. It has been
    plausibly suggested that “foobar” spread among early computer engineers
    partly because of FUBAR and partly because “foo bar” parses in electronics
    techspeak as an inverted foo signal; if a digital signal is active low (so
    a negative or zero-voltage condition represents a "1") then a horizontal
    bar is commonly placed over the signal label.