Grievous is an adjective that describes something that is extremely serious, severe, or causing great sorrow or pain. There are many synonyms for the word grievous that can be used interchangeably depending on the context. Some of these synonyms include dire, calamitous, catastrophic, disastrous, tragic, sorrowful, woeful, distressing, and melancholic. These words can be used to describe situations such as catastrophic accidents, devastating losses, or tragic events. They can also be used to describe personal experiences such as deep sadness or sorrow. When looking for a synonym for the word grievous, it is important to consider the context and choose a word that accurately conveys the severity of the situation.