'Ass high' is a stage of high greater than 'I'm feeling it', and greater than 'I'm high', but less than 'I'm fucked up'.
The term 'ass high' is often used in conjunction with the word 'Nigga' when referring to someone who is ass high.
person 2: "Shut up, you retard. If you need to go then just go."
person 1: "It feels kind of funny. I think I might of had some bad granola."
person 2: "Whatever... just don't go if you're ass high. It's too dangerous."