Sullen is an adjective that describes someone who is bad-tempered, sulky, and gloomy. There are various words that can be used instead of sullen, each with slightly different nuances and meanings. For example, a person who is brooding may be lost in thought or preoccupied with worries, whereas someone who is moody may experience sudden shifts in emotion. Another synonym for sullen is morose, describing someone who is constantly sad or depressed. Additionally, glum, surly, and dour are all good synonyms for sullen, offering different levels of severity to indicate a range of negative emotions and feelings. Choosing the right synonym can help to add nuance and depth to your writing or conversation, allowing you to more accurately describe someone's attitude or demeanor.