To brood means to think deeply and worry about something, usually in a gloomy or troubled way. When you brood over a mistake you made, you replay it in your mind again and again, dwelling on what went wrong. A student might brood about an argument with a friend instead of trying to fix it, or someone might brood over a bad grade rather than figuring out how to do better next time.

Brooding is different from simply thinking things through. When you brood, you're stuck in worried thoughts that loop around without leading anywhere useful. Picture someone sitting alone, staring out the window with a frown, lost in dark thoughts. That's brooding.

The word also has a completely different meaning related to birds and other animals. A hen broods her eggs by sitting on them to keep them warm until they hatch. During this time, she's called a broody hen. The baby chicks that hatch together are called a brood, whether they're chickens, ducks, or even alligators.

You might also hear the phrase “brooding presence” to describe someone who seems dark, mysterious, and thoughtful, like a character in a story who stands in the shadows, contemplating serious matters.