The Oxford English Dictionary says the exclamation has been spelled many different ways since it first showed up in 1604: “pue,” “peuh,” “peugh,” “pyoo,” and “pew.” The word is pronounced PYOO, but it's often stretched into two syllables for emphasis: pee-YOO.

How do you spell Pew EW?

pee-ew - Wiktionary.

Why does PU mean smelly?

P.U., or rather, PU, would appear to be an exaggerated version of peugh, previously pew, which mimics the disgusted exhalation of breath that often follows inhalation of a foul smell. Especially since puant and puer and putto would also seem to mimic that involuntary physical reaction.