1A prenominal adjective meaning unexpected or unlikely (used before nouns, e.g., 'an unexpected disaster').
とんだ勘違いで、二重予約になってしまった。
Because of an unexpected misunderstanding, there was a double booking.
とんだ災難に見舞われた家族を支援した。
We supported the family struck by an unforeseen disaster.
彼の一言でとんだ騒ぎが起きた。
His single remark caused an unexpected commotion.
2A prenominal adjective meaning terrible, awful, or outrageous (e.g., 'a terrible insult').
そんなことを言うなんて、とんだ失礼だ。
To say something like that is outrageous rudeness.
彼はとんだ嘘つきだとわかった。
It turned out he was a terrible liar.
3An obsolete adverbial usage meaning extremely or very (rare in modern Japanese).
（古風）とんだ喜びに涙が止まらなかった。
(Arch.) I was so extremely delighted that I couldn't stop crying.