In programming, F00 is what you call a variable with a vague name in which you store a color.
#F00 is the color code for bright red in simplified rgb hex. Foo is a variable name chosen by lazy programmers who can't come up with better variable names.
Plural: f00s
Verb: f00ing
var meh = "#FFF";
var iCantThinkOfWhatToNameThis = "#BADA55";
var F00 = "#F00";
var bar = "#123456";
Person 2: Stop f00ing your code up with so many f00s.
Derived from computer music (tracker) code. The "f" represents speed and "00" means that the song (actually pattern) stops.
So basically there's also my redefinintion for the rock band Foo Fighters... I don't think they're "fighting the fools", but rather "fighting to keep the music playing" or something in that direction ;)