It's recently occurred to me that the "Not" bit is really just a sarcasm-identify when you're too young to have a sense of sarcasm. The last time I ever babysat (a couple of months ago) the kid was not being fun or cool and refused to do her homework. So I told her, as a joke OBVIOUSLY, that if she didn't sit down and start, I would "lock her in the bathroom." Apparently this is not funny without the "Not!" Whatever.
I actually started saying "Not!" again, with my one-joke friend. You know those people that you had a class with, or worked with, or met at a party once, and you come up with this one inside joke. Then, every time you see them afterwards, its like, "Hey!! NOT!!" The one joke is never ever funny, and like you guys probably aren't really friends but you will literally pretend to be friends forever.
The "not" is really just for people who are absolutely retarded and have no sense of humor. Like if you read the title of my blog and thought, "Well that's nice" but if it was named "Living with my parents is cool....NOT!" you'd get it.