A sexually indiscriminate woman; a slapper. A conflation of the word jizz with '-oon', to evoke a spitoon, as it were, for glob. Like the cuspador, a place you can release problem fluids where spitting them at the wall isn't a graceful option.
I found half a dozen foil wrappers and a pack of gonorrhea pills in Trish's handbag again. That bitch is a Forrestal-class jizzoon.