The word "grumness" may not be an everyday term, but there are a variety of synonyms that can be used to convey a similar meaning. For instance, the word "grouchiness" is a common synonym that is used to describe a bad mood or a generally grumpy demeanor. Other synonyms for "grumness" include "surliness", "moroseness", "irritability", "sullenness", and "peevishness". These words all convey a similar tone of unpleasantness or ill-temper, and can be used interchangeably depending on the context. Whether writing literature or describing one's own emotions, any of these synonyms can add depth and nuance to language and communication.