From the jargon file:

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 coded so that a positive voltage or high current condition represents a "1", then a horizontal bar is commonly placed over the signal label.