These words, along with silly, simple, asinine, vapid, stupid, witless, and senseless, mean "lacking in judgment or intelligence, or both" and may ordinarily be used interchangeably. But they do have slightly different meanings and applications. A foolish person lacks both judgment and common sense and in addition may have a weak mind: "Eating heavily when you are not hungry is a foolish thing to do." "Your remarks are not only out of place but entirely foolish." Fatuous implies being not only dull and stupid but satisfied and complacent: "Because I have only one daughter, whom I adore, I realize that I am fatuous about her." "The lecturer haughtily provided fatuous answers to our questions." A foolish person cannot always help himself; a fatuous person usually can.
Dictionary of problem words and expressions. Harry Shaw. 1975.