Buzz
As fun as it would be, 'PU' doesn’t stand for 'Quite unpleasant!', 'Pungent, yuck!', or even 'Please use (deodorant)!'
In reality, it's not even an initialism. According to Grammarphobia, the exclamation probably comes from the 17th-century word 'pew,' which the Oxford English Dictionary defines as a sound expressing disdain, disgust, or mockery. The word has also appeared in various forms like 'pue,' 'peuh,' 'peugh,' and even 'pyoo.' While all these versions are typically pronounced as a single syllable, the prevailing theory is that people began to stretch it out into two syllables—'pee-YOO'—for extra effect.
As The Phrase Finder highlights, this is similar to how you might stretch out 'Bee-YOO-ti-ful!' instead of just 'Beautiful!' when admiring, say, a particularly fancy pigeon. Since 'Pee-YOO!' sounds so much like the letters 'PU,' it’s easy to see how it gradually became the version we use today.
That said, the idea of 'pew' evolving into 'PU' isn’t the only explanation. Some have proposed that it could stem from the Indo-European word 'pu,' which means 'to rot or decay,' or the Latin verb 'putere,' meaning 'to stink.' There are many words linked to 'putere' and other related Latin terms, such as 'putrere,' meaning 'to rot,' and 'puter' or 'putridus,' which both mean 'rotten.' These include words like 'pus,' 'putrid,' and the 16th-century term 'putor,' which refers to 'a bad or unpleasant smell.' All of these words, alongside others like the English word 'foul,' trace their roots back to the Proto-Indo-European '*pu-,' meaning 'to rot.'
In short, the letters 'pu' have long been connected with bad smells. As for whether the expression 'PU' directly originated from this or was coined in England (or elsewhere) at a later time, that remains uncertain.
