From LXF Wiki
When you don't know what you are, you've got yourself one of these. Most people do know who they are, though, even though most people aren't sane.
Apparently, wild Snooker legend Alex Higgins once went into a bar, splazzed up on drink. He exchanged some verbal with the barman, and then this happened:
Higgins: Do you know who I am?!
Barman: Hey folks, we've got a guy here who doesn't know who he is!
Suffice to say, Higgins was not amused. But he was great at Snooker so who cares, eh?