The term "most larkish" refers to something that is lively, cheerful and entertaining. Its antonym would describe something that is dull, serious and either quiet or sad. "Most larkish" suggests that the mood is light-hearted, but its antonym would indicate a more sober, somber atmosphere. It might also suggest a lack of energy or enthusiasm. Some possible antonyms for "most larkish" include gloomy, melancholy, bleak, morose, solemn, and heavy. These words describe a mood or atmosphere that is the opposite of lighthearted or cheerful, and convey a sense of seriousness, boredom and sadness.