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]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "in fun":
   as a joke, devilishly, elfishly, for amusement, for fun, for kicks,
   for laughs, for sport, impishly, in jest, in joke, in play,
   in sport, jestingly, jokingly, knavishly, mischievously, playfully,
   prankishly, puckishly, roguishly, scampishly, sportively,
   waggishly