To show how surprised or shocked you are. Or to describe something in a fun, positive way! This word can be used in the past, present, and future. For example..
Great wolf lodge will be so flabbergastful.
Great wolf lodge is so flabbergastful.
Great wolf lodge was so flabbergastful.
Disney was flabbergastful. Disney is flabbergastful. Disney will be flabbergastful