The Cambridge Dictionary defines understatement as "a statement that describes something in a way that makes it seem less important, serious, bad, etc. than it really is, or the act of making such statements". In the speech we see that Twain states that if you get offended by someone, you should not take extreme measures, just simply hit them with a brick whenever possible. We saw that "hit him with the brick" is the understatement, making the action seem less serious.
