(nonstandard) The quality of being fun.

Is the word funnest a proper word?

Funner & funnest

I am sad that "funner" and "funnest" are not proper words. They are lots of fun to use anyway. 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.

Can fun be used as an adjective?

Fun commonly functions as an adjective ("I had a fun time") and as a noun ("Let's have some fun"), and somewhat less commonly as a verb ("I'm just funning you").