The neck jizz refers to a subcategory of the strong pull-out game. It is a form of male ejaculation that may occur when you hit it from the back. Thereby, the jizz shoots out so paranormally strong, it accidentally lands on the neck instead of the booty.
"So how was your booty call with Dr. Baby?"
"Damn girl, he pulled out from behind and came so hard - it all landed on my neck."
"Holy fuck, I guess you received a classical neck jizz".