A metasyntactic variable used to represent an unspecified entity. If part of a series of such entities, it is often the first in the series, and followed immediately by bar.
From Chinese 福 ("fortunate; prosperity, goodluck"), via its use as 福星 in Chinese statues of the Three Lucky Stars, picked up from c. 1935 as a nonsense word in Bill Holman's Smokey Stover comic strip, whence it was picked up by Pogo, Looney Tunes, and others. Used by Jack Speer as the fannishghod of mimeography. Popularized in computing contexts by the Tech Model Railroad Club's 1959 Dictionary of the TMRC Language, which incorporated it into a parody of the Hindu chant om mani padme hum, possibly under the influence of WWII military slang FUBAR, which had been repopularized by Joseph Heller's Catch-22.