The word "boo" is often used as an expression of disapproval or to scare someone playfully. It's commonly heard in various contexts, such as when someone is trying to startle another person or to express dissatisfaction during an event like a performance or a sports game. In a broader sense, "boo" can also be used affectionately as a term for a loved one.
The word "boo" likely originates from the late 19th century, where it was associated with making a noise to startle or frighten someone. The precise origin is uncertain, but it resembles sounds made to express disapproval in various cultures.