Dumbfounded; struck speechless from shock or astonishment (often used as '唖然とする').
彼の告白にみんな唖然とした。
Everyone was dumbfounded by his confession.
その光景を見て、私は言葉を失い唖然とした。
Seeing that scene I was left speechless and dumbfounded.
新しい規則の内容を聞いて社員は唖然としていた。
When they heard the contents of the new rules the employees were stunned.
彼女の冗談に父は唖然とした表情を浮かべた。
Her joke left her father with a dumbfounded expression.