This is such a genius way to express disgust. It’s not a word that exists. It is not a word like “ewww” – which we recognize as disgust. It’s got an exhalation built into the word. It’s propulsive. That P sends it away and the ah dissipates it.
Shakespeare is genius for so many big things – big ideas, big metaphors, big images, big characters – but a sound like “pah!” is just as much genius as those bigger things.