restless, nervous energy
Turner is saying that the frontier didn’t just shape where Americans lived, but how they thought and acted. When he talks about traits like “individualism,” “restless energy,” and being practical and inventive he’s arguing that these attitudes came from having to survive and adapt on the frontier. Even though free land is gone, Turner thinks that mindset didn’t disappear with it. This helps explain why Americans still tend to push for new opportunities and resist limits, because expansion trained people to constantly look forward instead of settling into tradition.