Fun though it would be, PU does not stand for “Pretty unsavory!”, “Putrid, ugh!”, or even “Please use (deodorant)!”In fact, it’s not an initialism at all. As The Phrase Finder points out, this is not unlike how you might say “Bee-YOO-ti-ful!” instead of “Beautiful!” when you spot, for example, a fancy pigeon. Since “Pee-YOO!” sounds exactly like the letters PU, it’s not hard to believe that everyone eventually started thinking that’s how it was spelled. These include, among others, pus, putrid, and the 16th-century noun putor, meaning “a bad or unpleasant smell.”In short, the letters pu have been associated with stench at least as far back as ancient Rome. As for whether the expression PU came directly from there or arose in England (or somewhere else) much later, we can’t be sure.