Being a "LeeF" is when you either aimlessly stare off into space, are oblivious when someone is calling your name, have a constant runny nose, or act like you know what's going on when you really don't. You can catch yourself being a "LeeF" anywhere.
"Yo what's that girls name?"
"Um...I think number 10."
"You don't know what I'm talking about do you? Man your such a LeeF!"