From some American native speakers I've heard this as "Don't be a Foo!", it is usually reinforced by a barrage of expletives. It seems more common in urban areas, although heard in rural areas also where it is usually believed to be used in imitation of typically violent crime scene investigation dramas in American television shows.

Actually, the origin of "foo" comes elsewhere:

Foo and Bar Foo is the first metasyntactic variable commonly used. It is sometimes combined with bar to make foobar. This suggests that foo may have originated with the World War II slang term fubar, as a abbreviation for fucked up beyond all recognition. Foo was also used as a nonsense word in the surrealistic comic strip Smokey Stover that was popular in the 1940s and 1950s. See also Foo Fighters for more foo etymology.

Taken from: http://www.wikipedia.org/wiki/Metasyntactic_variable

Chuck

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