Thats a character from the Dragon Ball Z series. https://images.google.com/images?hl=en&q=buu&btnG=Search+Images
seriously speaking I think its just to boo someone (maybe in reply to a bad post or so (not that there are many good ones often))
its when you eat jalapenos loaded, mexican hot chilli with tabasco and peppers, that causes a hump to appear on your forehead, your lips to move to your left, and call out for a cool drink ... you can only say . buu