Sullen describes someone who is silently grumpy, withdrawn, and uncooperative in a way that's obvious to everyone around them. A sullen person doesn't yell or argue; instead, they sit with crossed arms, refuse to make eye contact, give one-word answers, and radiate displeasure through their body language and attitude.

Picture a kid who wanted to go to the park but has to do homework instead. Instead of protesting out loud, they slouch at their desk, stare at the page without working, and answer “fine” in a flat voice when asked how it's going. That's sullen behavior: a kind of passive resistance that communicates anger or disappointment without words.

The word often describes moods that settle in and linger. Someone might be sullen all afternoon after losing a game, or sullen for days after a disappointment. A sullen sky is gray and gloomy, looking like it might storm.

Sullen is different from simply being sad or quiet. Sadness might make you want comfort; sullenness pushes people away. It's a mood that says, “I'm upset and I want you to know it, but I'm not going to talk about it.”