Fun is a word which many people find does not live up to its definition, especially when it is used as an adjective (meaning “full of fun” or “amusing, enjoyable, pleasant”). In fact, there is a sizable portion of the English-speaking world which does not consider fun to be a proper adjective at all; these people feel that it is only a noun, which is why you sometimes hear that you should write “it was so much fun,” rather than “it was so fun.” It thereby stands to reason that if fun is not an adjective, it should not be used in the comparative and superlative forms of funner and funnest.
However, we do provide a definition for the adjectival use of fun, due to the fact that it has been used in such a fashion since at least the mid-1800s. We also list funner and funnest as comparative and superlative forms, due to the fact that these words are often employed in this manner (although we recognize that there are some who accept fun as an adjective, but feel that “more fun” and “most fun” are preferable). Including funner and funnest is not a directive that you should use these forms; it is a recognition of their existence, and you should feel free to use them or avoid them, as you see fit.
