Definition: More (or Most) Amusing or Enjoyable
I am sad that "funner" and "funnest" are not proper words. They are lots of fun to use anyway.
—User Comment on "Fun," Merriam-Webster.com
We may often use fun as an adjective today (‘I had a fun time’), but when the word first entered the English language at the end of the 17th century it was mostly used as a verb or a noun. In both of these cases the meaning had more to do with cheating or trickery than with enjoyment (to fun someone meant to trick them, and the noun form of fun was an instance of trickery or cheating).
In the middle of the 19th century the word shifted, and began to be used as an adjective. This change was not met with widespread approval, and to this day one may still find considerable numbers of people who insist that fun, no matter how many people might use it as such, is not properly an adjective. If, however, we accept that fun is an adjective, it seems reasonable that the comparative and superlative endings of -er and -est would be used. While more fun and most fun may still be the more common forms, there is evidence suggesting that a large number of speakers and writers of English are entirely comfortable with tacking an -er or an -est to this little word.
For even more information on "funner" and "funnest" see here: https://www.merriam-webster.com/words-at-play/are-funner-and-funnest-real-words-usage