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. According to Grammarphobia, the exclamation likely derives from the early 17th-century word pew, which the Oxford English Dictionary defines as “expressing contempt, disgust, or derision.” It’s also been spelled pue, peuh, peugh, and even pyoo. While all those iterations are technically pronounced as one syllable, the leading theory is that people drew it out over two syllables—pee-YOO—for added flair.
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.
That said, the pew-to-PU pipeline isn’t the only theory behind the expression. It’s also been suggested that it comes from the Indo-European word pu, meaning “to rot or decay”; or the Latin verb putere, meaning “to stink.” There are quite a few terms with ties to putere and other related Latin words (like putrere, meaning “to rot,” and puter or putridus for “rotten”). 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.
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