HISSING (6 letters) Voicing disapproval is a way of expressing one's dissatisfaction or disagreement with something. It can be done in a variety of ways, such as booing or HISSING. Booing is the act of making a loud, negative sound to show disapproval, while HISSING is the act of making a sharp, sibilant sound to express disapproval. Both of these are common forms of voicing disapproval and are often used in public settings.