A "jizz shart" is the after math of letting your partner finish inside you during anal. After anal sex, you will literally shart out the jizz.
Example: "Me and Eric had anal last night and he came in my ass. Shortly after, I kept jizz sharting."
"Me and Eric had anal last night and he came in my ass. Shortly after, I kept jizz sharting."
"I hate how after anal, you jizz shart."
When you have just had anal sex, you go to fart but jizz comes out.