What it means
To be flabbergasted is to be utterly astonished, knocked sideways by surprise. The word is a wonderful eighteenth century invention, probably a mash-up of flabby and aghast, and it sounds exactly as comically overwhelmed as it means. Reserved for the truly jaw-dropping, the news so big that gobsmacked alone will not quite cover it.
Usage examples
"I was absolutely flabbergasted when they announced my name, I had not prepared a single word and just stood there blinking."
"I was flabbergasted when they announced my name as the winner."
Where it comes from
An eighteenth-century coinage of murky make-up, probably "flabby" or "flap" jammed up against "aghast". The word sounds exactly like the feeling: jaw gone slack and flapping, utterly gobsmacked by something you never saw coming.
Other ways to say it
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