Question: What does "P.U." stand for?
Answer: It's a shortened term for puteo [also think of the word putrid], which is Latin for "to stink, be redolent, or smell bad." I actually called a professor of Latin at the University of Florida to verify this one.
It's actually spelled "piu," but is often pronounced as "pee-yew". It's root is the Indo-European word "pu," meaning to rot or decay. A lot of other languages use this root word and have the same general meaning.