Mirthless is an adjective that is commonly used to describe something that is lacking in amusement or joy. There are a number of synonyms that can be used in place of mirthless to convey the same meaning, such as joyless, cheerless, bleak, dull, drab, somber, and lifeless. Each of these words can be used to describe a situation, event, or person that doesn't bring any happiness or positivity. For example, a party that lacks energy or excitement can be described as mirthless or joyless. Similarly, a book or movie that fails to captivate or entertain readers or viewers can be considered bleak or lifeless.