A serious form of narcissism. To act as if one is an important person without actually doing anything worthwhile to prove it, thus making one look like a complete tool.
Sometimes abbreviated as USI.
Idiot: Hey guys! I just put up all these sweet pictures of myself on Facebook, and I've got this vlog that you've gotta check out. If you don't, you're clearly some kind of lesser being, 'cause everyone loves me for no reason!
Smart Person: Sounds like unwarranted self-importance to me.