The request left Reid “flabbergasted,” he says, but he agreed to take it on.
I am sitting there just flabbergasted and amazed the whole time.
Several senators at the hearing were flabbergasted at their answers.
1772, mentioned (with bored) in a magazine article as a new vogue word, perhaps from some dialect (in 1823 flabbergast was noted as a Sussex word), likely an arbitrary formation from flabby or flapper and aghast.
To amaze; perplex; throw (1772+)