To sulk means to stay quiet and withdrawn because you're upset, showing your bad mood through silence and a gloomy attitude rather than talking about what's bothering you. When someone sulks, they might sit alone with their arms crossed, refuse to join conversations, or answer questions with short, unfriendly responses.

Picture a kid who didn't get chosen for the lead role in the school play. Instead of congratulating the person who did or asking the teacher for feedback, they sit in the corner looking miserable, hoping everyone will notice how upset they are. That's sulking: broadcasting unhappiness through behavior rather than words.

Sulk can also be a noun, meaning a moody, resentful spell: “He's been in a sulk all afternoon.”