“surprised” や “shocked” 単独の意味とほぼ同様。
To be very shocked and surprised because something
It is used to describe an individual’s reaction of being ‘surprised’ or ‘shocked’ due to a pleasant or an unpleasant incident.
Mary was flabbergasted when her colleague put in her resignation.
I was flabbergasted on knowing about Sam’s death in a road accident.
You look very flabbergasted ! Do you have something to say ?
My mom was flabbergasted, when my dad told her that he had lost his job.
People are flabbergasted when they notice this beautiful lady has two extra fingers.
After seeing this wheelie stunt, I think wee all were flabbergasted.
John was absolutely flabbergasted to see the new Surface tablet by Microsoft featuring the new windows 8 OS.
The professor was flabbergasted to see students attending lectures on a Saturday early morning.
いっそセレナーデ / 井上陽水（バイオリン）