Grum is a slang term that is commonly used to describe a person who is grouchy or in a bad mood. However, there are several other synonyms that can be used in place of grum to describe a person's temperament. Some of these synonyms include irritable, cantankerous, surly, moody, crabby, and testy. Each of these terms describes a person who is easily agitated or annoyed, and may be prone to lashing out in frustration. Using these alternate terms can add variety to your writing and allow you to better convey the nuances of a character's personality or demeanor.