It's a combonation of the words "Poo And Eggs." It's literally the smell of shit and eggs from various things like treatment centers, or anything of the like.

One example of where one may smell pooneggs is at Eckerd College in St. Petersburg, Florida--very nice school, on the water, has a nice waterfront, gorgeous, gorgeous campus, but located very close to a treatment center, which makes the campus occasionally smell like shit and eggs.
Billy: "I go to Eckerd College."
Bob: "Oh really? How is it? I've heard it's an awesome school"
Billy: "It is but it smells like pooneggs."
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