Bob Ewell displays his lack of intellect and even common sense in the courtroom many times.
First of all, Ewell misjudges the situation. On the witness stand, he acts like a bantam rooster and struts up to the witness stand in the belief that he will demonstrate his racial superiority.
All the little man on the witness stand had that made him any better than his nearest neighbors was that. . . his skin was white (Chapter 17).
Certainly, Ewell's choice of language demonstrates his crudeness and ignorance as he describes his daughter screaming like "a stuck hog" and Tom Robinson supposedly "ruttin' on. . . Mayella" (Chapter 17).
As further proof of his dull wit, Bob Ewell did not consider what suspicions would be aroused because he did not call a doctor for Mayella.
Ewell displays his stupidity when Judge Taylor tells him to make no "audibly obscene speculations on any subject from anybody in the courtroom" (Chapter 17). He nods his head when the judge asks if he understands, but Scout says she does not think he understands the meanings of the words Judge Taylor uses, a judgment that proves true as Ewell uses very crude words right after being told not to do so. Again, the judge reprimands Bob Ewell for his language, telling him to remain within the "confines of English usage" (Chapter 17). Ewell mouths the words as he tries to figure out their meanings.
When Atticus questions him, Ewell does not understand the line of questioning. When Atticus asks Bob Ewell to write his name, the man has no idea why he is being asked to do this, but he is suspicious. When Judge Taylor remarks, "You're left-handed," Ewell becomes upset, saying he is a Christ-fearing man, and Atticus Finch is trying to trick him. Mr. Gilmer then asks Bob Ewell if he is ambidextrous. He answers, "I most positively am not. I can use one hand good as the others" (Chapter 17), which is the definition of ambidexterity.