"I fear nothing so much as a man who is witty all day long"

Madame de Sevigne's quote speaks to the concept that too much of an excellent thing can be a bad thing. In this case, she is referring to wit, or the ability to believe rapidly and cleverly. She is expressing her fear of somebody who is constantly amusing, as it could be a sign of a lack of depth or sincerity. She is suggesting that a person who is amusing all day might be hiding something, or might be attempting to cover up their true feelings. She is likewise suggesting that a person who is continuously witty might be attempting to manipulate or manage the discussion, which could be an indication of an absence of credibility. Ultimately, Madame de Sevigne is warning us to be careful of somebody who is witty all day, as it could be an indication of something more ominous.