Gerund; used as an uncountable noun meaning the act of astonishing.
stupéfactionNominalisation rare, au sens de l’action ou du fait de stupéfier. The flabbergasting of the board happened within minutes.
La stupéfaction du conseil d’administration s’est produite en quelques minutes.
flabbergastgerund / present participle Denotes something that causes great astonishment; often used before nouns.
It was a flabbergasting performance that left the crowd speechless.
C'était une performance stupéfiante qui a laissé la foule sans voix.