What to say to someone who claims "If people do not understand me, it is because they are stupid. If I do not understand them, it is because they were not clear"?

This is complete arrogance, gaavah. This person is showing that they have an ego problem and they think they're smarter than everyone else. Listen, the reality is when somebody speaks like this, they're revealing their own foolishness, not other people's. Let me tell you something. A truly wise person, a real chacham, knows how to explain complex things in a way that even a five-year-old can understand. If you're using big words and complicated language and nobody understands you except yourself, you're not proving you're smart. You're just proving you studied the dictionary. Chazal tells us in Pirkei Avot that chachamim have to be very careful with their words. Why? Because if you're really wise, you have to make sure that whatever comes out of your mouth is understood by the public. If this person doesn't understand others, it's not because they weren't clear. It's because this person isn't humble enough to listen properly. When you say "I know, I know, I know" before someone finishes speaking, you're showing that you have no manners and you're guaranteeing you'll never actually learn anything. You're stopping yourself from gaining knowledge because of your arrogance. The more ignorant people are, the more arrogant they become. This is a complete ridiculous oxymoron. The more ignorant you are, the more silent you should be. Gain knowledge, then speak. But people with this attitude are doing the opposite. You need to tell this person: if nobody understands you, the problem is with you, not with everyone else. If one person says you're not being clear, maybe it's them. But if everybody says they don't understand you, what does that mean? It means you're not communicating properly. You're assuming people have knowledge they don't have, or you're speaking in a way that's confusing, or you're just full of yourself. The biggest problem in every relationship is communication. People don't know how to express themselves clearly. They don't know how to be open, honest, and consistent. If you want people to understand you, you need to speak at their level, not above them. You need to assume they know nothing and give them the full picture. That's what a real teacher does. That's what someone with good middot does. Bottom line, tell this person that their attitude shows they lack basic humility and self-awareness. An intelligent person knows their limitations. When someone doesn't understand what you're saying, a wise person stops and thinks, maybe I need to explain this better. Maybe I need to clarify. Maybe I'm assuming they know something they don't. But an arrogant person just blames everyone else. This mentality of "everyone else is stupid" is a spiritual disaster. It prevents you from growing, from learning, from connecting with others, and from doing teshuvah. Nobody is going to listen to someone who thinks they're better than everyone. And if this person continues with this attitude, they're going to end up isolated and foolish, because they'll never accept guidance or correction from anyone. The person needs to work on their middot, especially gaavah. They need to understand that communicating clearly is their responsibility, not everyone else's. And when they don't understand someone, they should have the humility to ask for clarification instead of blaming the other person for not being clear enough for their "superior" intellect.