Adj. non-standard comparative form of "more fun". Meaning that a given something (noun) provides more mirth or more entertainment than something else.
This art museum is funner than the last three we went to.
Adj. non-standard superlative form of "most fun". Meaning that a given something (noun) provides the most mirth or entertainment than any other something.
The funnest way to contract diseases usually involve three women, two men, a broken condom, a goat, and a cheese grater.