I think the most un-American thing you can say is "You can't say that."

Keillor’s line treats free expression as a core American civic habit: the country’s identity is tied less to any single opinion than to the permission to voice opinions—even unpopular, impolite, or dissenting ones. By calling “You can’t say that” the “most un-American” remark, he frames speech-policing as a kind of cultural betrayal, suggesting that social or institutional attempts to forbid utterances undermine democratic pluralism. The quote also implicitly distinguishes between criticizing speech (which remains speech) and prohibiting speech (which narrows the public sphere). Its punch comes from reversing expectations: the truly “un-American” act is not saying the wrong thing, but declaring certain things unsayable.