Literally, an old sock used to clean up one's ejaculate after masturbation or sex. Sometimes, the sock is used during masturbation as well.
Also, an insult for an individual perceived as being as worthless as a jizzed-on sock.
I woke up next morning in the office of the strip club I work naked holding a Jizz Sock in one hand and a bottle of Grand Mariner in the other.
That hack comedian stole his whole act from Richard Pryor and Comic View. What a Jizz Sock!
If you've made use of the term "wet blanket" you understand that a wet blanket is someone that is just annoying, in the way, ruins the mood, etc. They're a wet blanket. No one wants a wet blanket. How can that keep you warm? How can it provide any kind of comfort? It can't. It sucks.
Out of the womb of the wet blanket came the term "jizz sock". A jizz sock is a slutty wet blanket! When you're jerking off, you just grab any old jizz sock off the floor and then throw it in the laundry bin. No one likes a jizz sock once it's been used.
Jennie: You know how you explained to me what a wet blanket is?
Kelly: Yes I do!
Jennie: Well, I thought about slutty wet blankets... why don't we call them jizz socks?
Kelly:... That... That needs to be put on urban dictionary!!! 'Yea she's a fucking jizz sock... just some shit that was lyin' around on the floor' It's PERFECT.
When you jizz in your condom after having sex
Girl: what's that?
guy: it's my jizz sock, sock waste : cum