Most crack-up is a slang term that refers to something or someone that is extremely funny, hilarious or amusing. However, every word in the English language has its opposite or antonym, and most crack-up is no exception. The opposite of most crack-up is something or someone that is dull, boring, unamusing, or unfunny. Other antonyms for most crack-up include tedious, unexciting, dull, and tiresome, among others. So while most crack-up is an excellent way to describe something that can make you laugh uncontrollably, it is essential to remember that its opposite exists and can also be useful in describing things that are not funny.