Antonyms for the word "larkish" include serious, solemn, grave, austere, formal, and restrained. While "larkish" refers to something that is playful, carefree, and full of fun, its antonyms express a completely different mood and tone. Seriousness and solemnity connote a sense of gravity and weightiness, while restraint and formality indicate a reserved and composed demeanor. Austere implies a lack of adornment or ornamentation, while gravity refers to a sense of depth and seriousness. Whatever the opposite of "larkish" may be, it is clear that it evokes a much different feeling than that of its original term.