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dumbfounded; speechless; flabbergasted; struck dumb with astonishment
彼の発言に唖然とした。
I was dumbfounded by his remark.
唖然として言葉が出ない。
I was speechless and dumbfounded.
予想外の発表に唖然としてしまった。
I was left dumbfounded by the unexpected announcement.