Grum is an adjective that means sullen or gloomy. 

The origin of our word of the day is in the Old English period where it meant ‘fierce.’ In time, grum came to take on a meaning similar to words like ‘morose’ and ‘surly.’ Here’s an example:

Tommy’s grum demeanor told me it would be a tough day for anybody who tried to talk to him. When he gets in a foul mood, it’s best to stay away.