From LXF Wiki

The single most popular 'junk' programming variable. Part of 'foo bar', a corruption of 'fubar', and generally goes along with 'baz' to give three three-letter words that are used for demonstrative purposes, eg:

function foo(int bar, char* baz);

Foo is also remarkable as being the name Mr T used for people he didn't like, viz: 'I ain't going on no aeroplane, foo'.