This is a serious question. I can't find it in the dictionary.
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Actually in an interview done with Dave Grohl in 1996, he explains that the word "Foo" in Foo-Fighters, refers to "Fire" 


"Foo fighter" was supposedly used initially as a semi-derogatory reference to Japanese fighter pilots (known for erratic flying and extreme maneuvering), but it became a catch-all term for fast moving, erratically flying objects. There were several other terms used to describe these objects, such as "Kraut  fireballs", but "foo fighter" seems to have been the most popular.

The term is generally thought to have been borrowed from the often surrealist comic strip Smokey Stover .[1] Smokey, a firefighter , was fond of saying, "Where there's foo there's fire." (This "foo" may have come from feu, the French  word for "fire ", or Feuer the German  word for "fire ", or from Smokey's mispronunciation of the word "fuel".)
yes...foo means food in malay and chinese
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In Russian language foo mean that something smell stinky