"The Ransom of Red Chief" by O. Henry is a story replete with ironies, starting with the first line of the story:
It looked like a good thing: but wait till I tell you.
What seemed like a good idea was not, and that will be the central theme of the story. Even the town where this story takes place is part of the irony. One of the two men who will soon become a kidnapper is Sam, and he describes the town where they decide to do their kidnapping as being "flat as a flannel-cake" (a pancake) and yet the town is called Summit. And the two men who seem to be pretty smart are not.
One of the first and most obvious ways the author tries to make the men seem smart is through their vocabulary, though if we are paying attention we would know that even that is not quite right. In the first paragraph, for example, Sam quotes Bill as saying that this event took place "during a moment of temporary mental apparition," which is not a sensible statement. He probably meant something more like "mental aberration," but who knows.
Sam does seem to have a little better understanding of the big words he is using. He says this of the town:
It contained inhabitants of as undeleterious and self-satisfied a class of peasantry as ever clustered around a Maypole.
At least "undeleterious" (which means harmless) is a sensible word in this setting; however, "self-satisfied" means they are exceptionally content with themselves, which may be true but seems an odd thing to notice about the inhabitants of a town (especially for such unobservant men). A few sentences later, Sam's propensity (tendency, persistent habit) for using big words grows ridiculous when he describes "semi-rural" communities as having a strong "philoprogenitiveness," which simply means they have a lot of children.
As the story progresses, we discover that the two kidnappers, who seem to have a plan and seem to be more intelligent than most (or at least some) are actually rather ridiculous and bumbling fools. Almost immediately after they capture their young victim, the tables are turned and the men become the almost-literal captives of a nine-year-old named Johnny who has become "Red Chief, the terror of the plains."
O. Henry, then, uses the men's sophisticated, complicated vocabulary to create more irony about the men's lack of sophistication in nearly everything they do. While their words indicate that Sam and Bill are smart, their actions, ironically, certainly prove otherwise.