The pen is mightier than the sword" is a metonymic adage, penned by English author Edward Bulwer-Lytton in 1839, indicating that communication, or in some interpretations, administrative power or advocacy of an independent press, is a more effective tool than direct violence.
In this situation, Huck is trying to be impressive. He is trying to get the point across that he is funny and "cool". This lie is not only the one of a kind for Huck, but also one where he tries to push his reputation past its limits.
