Yurusanai or yurusenai is often used in manga and anime. Often said with a similarly angry tone, it can be confused with urusai, but it is a completely different word with a very different meaning.
Yurusanai is often translated as “I won’t forgive you”. This is a reasonable translation in some cases, but often falls rather short of the full meaning.
The term is often heard when a hero makes a stand against a villain who is doing something unforgivable. However, the sense of the phrase in this case is often closer to:
I won’t let you do this.
The reason for the difficulty is that word 許す yurusu means both “to forgive” and “to allow” and also has an implication of “to give up”. So that shouted yurusanai (the negative form of yurusu) means at once “I won’t forgive you” and “I won’t let you do this” with overtones of “I won’t give up”.
This makes it a very powerful expression in these circumstances, and one that has no brief and direct English translation.
When a word combines several meanings, those meanings are often closely entwined in the mind of the speaker. You may have noticed that Germans speaking English sometimes say “happy” when they mean “lucky”. That is because the German word Gluck means both happiness and luck, so that the two concepts are more closely bound up in the German mind than in the English.
The same is true of the concepts of allowing and forgiving in the word yurusanai. The resulting mixture gives a powerful expression in the negative, which is why it is so often used.
Sometimes you will hear yurusenai in place of yurusanai. The only difference here is that yurusenai means “I can’t allow/forgive” rather than “I won’t”.
Because it tends to be spoken in anger, the word is usually used in the plain form. However, there are occasions when it is used in the polite form, sometimes to great effect.
Cure Beauty’s first appearance in Smile Precure is prefaced by her ojousama civilian persona facing down an evil witch who has downed all the current Precures.
Immediately before her debut transformation, she makes a defiant but dignified speech ending with the words:
watakushi, Aoki Reika ga yurushimasen.
I, Reika Aoki, will not forgive you/allow this.
This unusual (for anime) use of yurushimasen, the polite negative of yurusu, gives a powerful and dignified effect.