The word "underlip" refers to the lower lip, or the part of the lip that is situated below the mouth. However, there are several other terms that can be used to describe this facial feature. These include terms like bottom lip, lower lip, sublabial region, inferior labial region, and mandibular lip. Each of these terms has a specific use and can be used interchangeably depending on the context of the sentence. Whether you're writing a medical report, a romantic poem, or a descriptive passage, there are plenty of synonyms for the word "underlip" that can help you communicate your message more effectively.