Foolish people are silly or senseless, and when you do something foolish, it's clearly unwise or irrational. Sitting on the hood of a car while your friend is driving is a foolish thing to do.
Things that show a lack of judgment are foolish, like cheating on a test or running into the street after a basketball. Foolish decisions at work can cost you your job, and foolish comments can hurt people's feelings. Foolish is a 14th century word that comes from fool, a person who's unwise. The Latin root, follis, means "bellows" or "leather bag." The "silly person" meaning comes from the figurative idea of a "windbag."