When a guy bangs a hot chick or beats off to amazing porn he shoots his load. It could be used as a joke too. As to insult someone or to threaten them.
Oh my, last night I banged some chick so hard I shot my load right into her mouth.
Now, as a joke:
Punk: Hey, screw you dude.
Cool kid: Dude no, I'm gunna shoot my load right into your cereal