Means any person named Jess or Jessica will not do anything you try and make her do.
Patch - " Hey Jess pass me the ketchup."
Jess - " Not Jess."
Used almost the same as "I ain't goin." Means you will not do something and there's no way you will let anyone talk you into or make you do it.
Alex -" Hey chug those 3 four lokos!"
Patch - "Not yung"