A Jizz Monkey is a stuffed animal, usually a Monkey plush with a slight slit cut into the bottom of the Monkey creating a hole for a man's weiner.
Greg: Dude why do you have this old crusty Monkey on your bed?
Colton: Dude don't touch that, that's my Jizz Monkey
Greg: Oh I have one too but it's pink not brown.