FUNNER
Adjective
funner
(nonstandard) comparative form of fun
Usage notes
While funner is a regular comparative of the adjective fun, the comparative more fun is much more common. The use of fun as an adjective is itself still often seen as informal or casual and to be avoided in formal writing, and this would apply equally to the comparative form. Merriam-Webster gives fun as an adjective without comment, and states that funner and funnest are "sometimes" used.
Source: Wiktionary
FUN
Fun, n. Etym: [Perh. of Celtic origin; cf. Ir. & Gael. fonn
pleasure.]
Definition: Sport; merriment; frolicsome amusement. "Oddity, frolic, and
fun." Goldsmith. To make fan of, to hold up to, or turn into,
ridicule.
Source: Webster’s Unabridged Dictionary 1913 Edition