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]

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]

WordNet (r) 3.0 (2006):

fun
    n 1: activities that are enjoyable or amusing; "I do it for the
         fun of it"; "he is fun to have around" [syn: fun,
         merriment, playfulness]
    2: verbal wit or mockery (often at another's expense but not to
       be taken seriously); "he became a figure of fun"; "he said it
       in sport" [syn: fun, play, sport]
    3: violent and excited activity; "she asked for money and then
       the fun began"; "they began to fight like fun"
    4: a disposition to find (or make) causes for amusement; "her
       playfulness surprised me"; "he was fun to be with" [syn:
       playfulness, fun]

The Free On-line Dictionary of Computing (19 January 2023):

Fun

   A typed lambda-calculus, similar to SOL[2].  "On
   Understanding Types, Data Abstractions and Polymorphism",
   L. Cardelli et al, ACM Comp Surveys 17(4) (Dec 1985).