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PU is an exaggerated pronunciation of pew, an interjection that is appropriate when you smell something foul.
IIRC, thiols are the chemicals that make things like poop and rotting stink so bad, and the human nose is really sensitive to them - we can smell something like 10 parts per billion. I can find cites for skunk spray containing thiols, but nothing on what else thiols are in, just a bunch of scientific stuff about them.