Learn German: Auf keinen Fall!

Literally, it translates to "On no case!" The word der Fall means "the case" or "the situation." In German, this is the standard, idiomatic way to say "Under no circumstances!" or "No way!"

Why is it keinen instead of just kein?

In this specific fixed expression, the preposition auf triggers the accusative case. Because der Fall is a masculine noun, the negative article kein must take the masculine accusative ending, becoming keinen.

Can I use a similar phrase to say the exact opposite, like "Definitely!"?

Yes! The opposite phrase is Auf jeden Fall!, which literally means "On every case!" It is extremely common in everyday German to express "Definitely!", "By all means!", or "In any case."

Is this just a longer way of saying Nein?

It is much stronger than a simple Nein. While Nein just means "No," using Auf keinen Fall! adds heavy emphasis, much like saying "Absolutely not!" or "Not in a million years!" in English.