What does the quote by Fran Lebowitz "Polite conversation is rarely either" mean?
Does it mean "If it's polite it's likely not a conversation, if it's a conversation it's likely not polite?"
Polite conversation is rarely either
I would say it means: A "polite conversation" is usually neither polite nor a conversation.
Basically, if someone sees the need to characterize the discussion they have with someone else as "polite conversation" then they were probably being confrontational. You wouldn't casually say of a friendly chat, "I just had a polite conversation with my good friend Fred," but instead would use other words to characterize the discussion.