Ploo is a response to any type of question, answer, or statement. It can be positive or negative, and is usualy delivered when you are uncertain of what to say. Therefore It is the perfect response to anything said to you. (Unless that anything is "I'm Pregnant" in which case instead of saying "Ploo" you should run or deliver a swift punch to the ovaries, killing the unborn child thus eliminating your problem.)
"Yo, sorry i cant go to the movies tonight"
" Well, ploo."