The Fifty-First Dragon (1954)

Based on the Heywood Broun story of an awkward Knight who couldn't keep up with his classmates at the Killing Dragons Knight School, until a professor, in order to instill self-confidence in the Knight, convinces him that he can kill dragons with the best of them if he uses the magic word. And so he does until he meets his fifty-first dragon and forgets the magic word, and loses his self-confidence crutch provided by a word.