Pooh is an interjection used to express disdain, contempt, or disbelief. There’s also the informal verb pooh-pooh, which means to express contempt for or to speak ill of. In today’s English, poo refers to defecation and the product of defecation. Both words are also terms of endearment, usually applied to small children or pets.
Pooh-pooh is inflected pooh-poohed, pooh-poohing, and pooh-poohs. Poo makes pooed, pooing, and poos. There is a tendency to insert an apostrophe or a hyphen in these forms—e.g., poo’ed, poo-ing—but there’s no good reason for doing so. Other verbs ending in oo are inflected with no such mark—e.g., ballyhooed, shooing, tattooed.
Pooh goes back several centuries. It probably began as a written approximation of a sound one might make when expressing contempt. The OED lists several similarly aged interjections—including pew and poh—that likely came from the same source.
Poo is newer. At least one source calls it a children’s word from the 1950s. This date is questionable, though, as presumably poo is closely related to poop, whose excrement-related senses came about much earlier.
A Stoke resident says she has fears about a dog poo dumping campaign after news of a second site of dog poo bag debris emerged. [Nelson Mail]
Oh, pooh. Now you know the plot of the whole movie. [Virginia Pilot]
Charles Darwin pooh-poohed that idea, pointing out that zebra graze in the open. [The Economist]
Then a pigeon pooed from the tree above, narrowly missing us. [Guardian]
They don’t even function as a helpful guideline, with most pooh-poohing the very notion of common sense. [News24]
Nope, it’s not the prospect of unfeasibly daft blokes getting boozed and pooing in hotel corridors. [New Zealand Herald]