i-adjective
unthinkable; outrageous; absurd
Used to describe something so extreme or unexpected that it is hard to believe or accept. Often carries a negative or shocked tone.
彼がそんなことを言うなんて、とんでもない話だ。
It's an outrageous story that he would say such a thing.
とんでもない
I made an unthinkable mistake.
interjection
absolutely not!; no way!; what a thing to say!
Strong interjection rejecting a suggestion, accusation, or compliment. Expresses disbelief or strong denial.
「まさか君がやったんじゃないだろうね？」「とんでもない！」
"You didn't do it, did you?" "Absolutely not!"
interjection
not at all; don't mention it; no bother at all
Humble interjection used to deflect thanks or apologies, indicating that the action was no trouble. Polite and modest.
「手伝ってくれてありがとう。」「とんでもない。いつでもどうぞ。」
"Thank you for helping." "Not at all. Anytime."