From The Collaborative International Dictionary of English v.0.48:
Fun \Fun\ (f[u^]n), n. [Perh. of Celtic origin; cf. Ir. & Gael.
fonn pleasure.]
Sport; merriment; frolicsome amusement. "Oddity, frolic, and
fun." --Goldsmith.
[1913 Webster]
To make fun of, to hold up to, or turn into, ridicule.
in fun, jokingly; not seriously; for amusement.
like fun, definitely not; -- used adverbially, mostly to
contradict a prior assertion; A: "He says he won't do it
again." -- B: "Like fun he won't.".
[1913 Webster + PJC]
From The Collaborative International Dictionary of English v.0.48:
Fun \Fun\ (f[u^]n), adj.
Of or pertaining to fun; causing pleasure or amusement.
[informal]
[PJC]