Antonyms for the phrase "more knee slapper" include expressions such as dull, boring, uninteresting, tedious, and unamusing. These words have the opposite meaning to the phrase, which denotes something hilarious or funny that causes one to laugh so hard that they slap their knees in reaction. Other antonyms for this phrase that carry similar weight include humorless, gloomy, heavy, serious, and somber. It is important to note that antonyms are essential in helping people understand the meaning of a word or phrase by highlighting its opposite meanings, and in this case, these antonyms provide a contrasting perspective to the phrase "more knee slapper.