Answer:
Scout. She said this because she was trying to talk to Mr. Cunningham, and she had been told by Atticus that talking to people about their interests is polite, so when she remembered this, she tried talking about Walter, but because Mr. Cunningham showed no interest to this, so she tried talking about entailments, how they were bad, specifically. Everyone kind of stared at Scout in awe because she had even brought up entailments. (Entailments are when the owner of property has limited power over that property.)