The act of simultaneously pooping and ejactulating. Term comes from the combination of the wordsjizz and feces. This usually happens when something is so exciting and/or arousing that it causes one to not only ejaculate, but also shit oneself. Fizzing can often be accompanied by loud outbursts, moaning, and hurried trips to the bathroom.
Jon: "Wow, did you seeRonaldo's goal last night?"
Bob: "Yeah, it was awesome. I saw it on Sportscenter and started fizzing."