A backup, ie. refering to a person as the justification for doing something.
This copy is for Justin Case.
The name of a fictional person, which is used for the reason that you are doing something. Used in place of "just in case".
bob: can you pack an extra sleeping bag?
mary: why? we have enough...
bob: for Justin.
bob: Justin Case.
Jack: just chuckin in a few extra beers for Justin...
to be the back up guy/girl when dating someone
Dude, you remember that chick I went out with last week?
I'm pretty sure that whore already has a boyfriend. I think I'm her Justin Case guy if they break up.