The phrase "killingly funny" is often used to describe something that is incredibly humorous. If you are looking for synonyms for this phrase, some options include side-splittingly funny, rib-ticklingly hilarious, gut-bustingly amusing, uproariously entertaining, and knee-slappingly comical. These phrases all communicate the same level of hilarity as "killingly funny" but provide some variation in language to keep your writing fresh. No matter which synonym you choose, always make sure to use descriptive language and vivid imagery to convey the full impact of the humor you are describing.