taru-adjective, adverb taking the 'to' particle
dumbfounded; speechless; flabbergasted
Describes a state of being so shocked or astonished that one is left speechless. Often used in the adverbial form 唖然として or as a たる adjective in literary contexts.
彼の突然の告白に、私は唖然として何も言えなかった。
At his sudden confession, I was dumbfounded and couldn't say anything.
そのニュース
Hearing the news, everyone had a speechless expression.
Mixed kana-kanji spelling sometimes used to avoid the non-jōyō 唖.
Compound of 唖 (a, 'mute, speechless') and 然 (zen, a suffix forming adjectives/adverbs meaning 'in the state of'). The exact historical derivation is uncertain, but the word has been used in literary Japanese to describe a state of speechless astonishment.