intr.v.
funned, fun·ning, funsInformal To behave playfully; joke. adj.
Informal Enjoyable; amusing: "You're a real fun guy"(Margaret Truman).
[Possibly from fon, to make a fool of, from Middle English fonnen, to fool, possibly from fonne, fool.]
Usage Note: The use of fun as an attributive adjective, as in a fun time, a fun place, probably originated in a playful reanalysis of the use of the word in sentences such as It is fun to ski, where fun has the syntactic function of adjectives such as amusing or enjoyable. The usage became popular in the 1950s and 1960s, though there is some evidence to suggest that it has 19th-century antecedents, but it can still raise eyebrows among traditionalists. The day may come when this usage is entirely unremarkable, but writers may want to avoid it in more formal contexts.
Also, in fun. As a joke, not seriously. For example, For fun the children told the teacher it was a holiday, or Their teasing was just in fun. [Mid-1800s]
for the fun of it; . For pleasure or excitement. For example, He played basketball for the fun of it, or They drove around for hours, just for kicks. Also see for the hell of it.
[fuhn] 
noun, verb, funned, fun⋅ning, adjective 