In the book "Alice's Adventures in Wonderland", Alice encounters a "Cheshire cat". At the end of the conversation, the cat disappears slowly, until only its smile is left, in mid-air. Then the smile disappears too.
In ordinary usage, people say that someone is "grinning like a Cheshire cat" when they're smiling broadly (or enigmatically).
"Flabbergasted" means "absolutely astonished".