in fun

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]
    

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