To hiss is to make a long s sound. Snakes are known to hiss, and sometimes unhappy audience members will hiss instead of booing.
The sound you make when you hiss — also called a hiss — sounds just like the word itself. In fact, you can't say the word without hissing. This type of sound is also known as sibilance, a consonant that's pronounced when a stream of air passes through teeth that are touching or close together. A barista's steaming wand hisses as she foams milk, and some types of snakes hiss in warning.