とんでもない

From 途+でも+ない. Similarly to 途方もない, 途 (way) means reason, and the whole is "does not even follow (the way of) reason".

とんでもない (-i inflection, rōmaji tondemonai)

1. unbelievable, unreasonable

   俺 (おれ)はとんでもないことをしてしまった。

   Ore wa tondemonai koto o shiteshimatta.

   I did something totally outrageous.

   • 1983 November 20 [Nov 15 1981], Murakami, Motoka, “オラは日にっ本ぽん一になるの巻まき [Ah’ll be Japan’s Number One]”, in 六ム三サ四シの剣けん [Musashi’s Sword], volume 1 (fiction), 11th edition, Tokyo: Shogakukan, →ISBN, pages 85–86:

     栄 (えい)一 (いち)郎 (ろう)君 (くん)と佳 (か)代 (よ)さんのあいだに生 (う)まれたら、まさに剣 (けん)の申 (もう)し子 (ご)というところですなあ、六 (ム)三 (サ)四 (シ)君 (くん)は…………東 (とう)北 (ほく)の鬼 (おに)ユリ、天 (てん)下 (か)無 (む)敵 (てき)の女 (じょ)性 (せい)剣 (けん)士 (し)として鳴 (な)らした佳 (か)代 (よ)さんまで入 (にゅう)門 (もん)していただけるとは、光 (こう)栄 (えい)ですな。

     Eiichirō-kun to Kayo-san no aida ni umaretara, masa ni ken no mōshigo to iu tokoro desu nā, Musashi-kun wa………… Tōhoku no Oniyuri, tenka muteki no josei kenshi to shite narashita Kayo-san made nyūmon shite itadakeru to wa, kōei desu na.

     Musashi-kun is Eiichirō-kun and Kayo-san’son. I have no doubt he’s a natural swordsman………… Kayo-san was the renowned unmatched female swordsman, the Oni Lily of the Northeast. It’d be an honor to have you help us with the newcomers, too.

     とんでもない、先 (せん)生 (せい)……私 (わたくし)なんてもう全 (ぜん)然 (ぜん)ダメですわ。六 (ム)三 (サ)四 (シ)を生 (う)んで以 (い)来 (らい)、すっかり太 (ふと)っちゃってお腹 (なか)の肉 (にく)がつかめちゃう〰〰〰！

     Ton de mo nai, Sensei…… Watakushi nan te mō zenzen dame desu wa. Musashi o unde irai, sukkari futotchatte o-naka no niku ga tsukamechau~~~!

     I’m not sure about that, Sensei…… I’m completely out of shape now. I’ve only accumulated belly fat ever since I gave birth to Musashi〰〰〰!

2. not at all

• とんでもなく (tondemonaku): dreadful
• トンデモ (tondemo-): (prefix) outrageous, crazy