An Illusion is the state of being deceived. Grandeur is the quality of being impressive or awesome. Therefore, someone who has Illusions of grandeur is deceiving themselves into thinking they are impressive or awesome in some way. In reality, no one else sees them in this way.
Ann Weiss thought she knew what she was talking about - like she was some authority on English or something - but in reality, she was having illusions of grandeur.
Prices shown in USD.