"foo" origin

[Pat O'Connell]

In an early Daffy Duck cartoon (late 30's?--it's in black & white) that my
kids replay incessantly, they use the phrase "Silence is Foo" to ask for
quiet in the operating room theater while Daffy's mad doctor boss sews up
a football.  So the expression is older than WW II--I think it means
something like the "mandatory" rather than "fouled up".  Anyway, my 
daughter asked about what it means in the cartoon.  Anyone out there of
appropriate vintage to help us figure out this slang??

Pat O'Connell