"No way!" "Not at all!" "Unthinkable!" "What a thing to say!" and "Don't mention it".

Used as a single phrase: "Not at all!", "Don't be absurd!", "It's no problem at all!" Used with a noun such as こと: "Something unthinkable!", "something incredible!", etc.

Another useful expression could be: そんなことないよ！ I hear that translated as "It's not like that at all!"

in the dicanory it says that the world "tondemonai" its an adj. 1. adj. means horrible, suprising. 2nd adj. unreasonable. 3. interj. my pleasure! 4. interj Not a chance.