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unthinkable; unexpected; absurd; outrageous; preposterous; terrible
absolutely not!; far from it!; impossible!; preposterous!; what a thing to say!; no way!
(it was) no bother at all; not at all; don't mention it
とんでもない事件が起きた。
Something outrageous happened.
とんでもない、お気になさらず。
Not at all, don't worry about it.
深夜に届いた請求額はとんでもなかった。
The bill that arrived late at night was outrageous.