Gerund; used as an uncountable noun meaning the act of astonishing.
Definitions
The act or process of astonishing someone. The flabbergasting of the board happened within minutes.
flabbergastgerund / present participle Denotes something that causes great astonishment; often used before nouns.
Definitions
Extremely surprising or astonishing; astounding. It was a flabbergasting performance that left the crowd speechless.