Thighslapper is a term used to describe something that is extremely funny. However, there are a number of different synonyms that can be used in place of this slang term. For instance, the phrase "belly laugh" can be used to describe something that is incredibly amusing and causes one to laugh hysterically. Another term that can be used is "side-splitting," which refers to something that is so funny that it feels as though one's ribs are being pulled apart. "Rib tickler" is another alternative, as is "knee-slapper," which refers to the physical reaction that people often have when they are laughing uncontrollably.