Phat boy Share definition Flag noun; a fat piece of shit that thinks he knows how to play percusion (usually from mexico) hey dude, look. Not is that guy acting like a douche bag but he is also being a Phat boy Phat boy by kevo_jsr September 20, 2007 0 0 Get merch Get the Phat boy mug. Buy Now — $32.95 See more merch