The antonyms for the word "most kneeslapper" would be phrases like "least funny," "most unfunny," or "least amusing." These terms indicate that the subject or object being described is not humorous or fails to elicit laughter. Other antonyms could include "most serious," "most somber," or "least playful," which connote a lack of levity, playfulness, or lightheartedness. While the phrase "most kneeslapper" implies that something is extremely funny, the antonyms suggest that the opposite is true, and that the subject is dull or serious. Using these antonyms can help provide a more nuanced or descriptive analysis of a situation or experience.