A metasyntactic variable is a placeholder name used in computer science, a word without meaning intended to be substituted by some objects pertaining to the context where it is used. The word foo as used in IETF Requests for Comments is a good example2.
[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 label3.