1. a delusion (common in paranoia) that you are much greater and more powerful and influential than you really are
2. what occurs when you have feelings for someone that are so extreme, and you describe them, but the other person doesn't feel the same way... at all.
Originating from the notoriously bad playstation user, Granteus, this is an expression for somebody who is extremely horrible at something. It is often abbreviated to Grant.