Page "Knights who say Ni" Paragraph 4

The Knights happen to have a weakness in that a number of words, when spoken to them, has the same effect on them as their saying, " Ni " has on others.

The only one of these words that is revealed in the film is " it ," which is picked up on when Arthur, upon being asked to cut down the mightiest tree in the forest with a herring, declares that " it can't be done.

" Ultimately, King Arthur and Sir Robin ( who shows up with his own knights ) both say " it " several times in conversation ( unaware that " it " was the word that was causing the Knights ' pain.