To hiss means to make a sharp, prolonged “sssss” sound, like air escaping from a balloon or water hitting a hot pan. Snakes hiss as a warning when they feel threatened. Cats arch their backs and hiss when they're angry or scared.

The sound itself feels menacing and unfriendly, which is why people also hiss to show disapproval or contempt. When a villain appears in an old-fashioned play, the audience might hiss to show they don't like the character. If someone hisses an insult at you, they're saying it in a harsh whisper, usually through clenched teeth, trying not to be overheard but still wanting you to feel their anger.

As a noun, a hiss is the sound itself: the hiss of steam from a radiator, the hiss of a snake in the grass, or the hiss of air brakes on a bus. The word captures both the sound and the feeling it creates, which is usually tension, danger, or hostility.