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]

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "fun":
   act, action, amusement, amusing, animal pleasure, ball,
   be merry with, beguiling, behave, belly laugh, big time,
   blitheness, blue story, bodily pleasure, carnal delight, comfort,
   content, contentment, coziness, crack a joke, crack wise,
   creature comforts, delightful, dirty joke, dirty story, disport,
   diversion, diverting, do, double entendre, ease, endpleasure,
   enjoyment, entertaining, entertainment, ethnic joke, euphoria,
   fleer at, fool, forepleasure, frivolity, fruition, fun and games,
   function, funmaking, funny story, gag, game, gibe at, glee,
   gleefulness, go, good one, good story, good time, gratification,
   great fun, great satisfaction, gusto, hearty enjoyment, high glee,
   high old time, high time, hilariousness, hilarity, howler,
   humorous, intellectual pleasure, jape, jest, jestbook, jive,
   jocularity, jocundity, joie de vivre, joke, jolliness, jollity,
   jolly, josh, joviality, joy, joyfulness, joyousness, keen pleasure,
   kicks, kid, kid around, laugh, laughs, laughter, levity,
   lovely time, luxury, make a funny, make fun, make fun of,
   merriment, merriness, mirth, mirthfulness, mischief, mock,
   officiate, operate, panic, perform, physical pleasure, picnic,
   play, play on words, pleasant time, pleasure, point, poke fun at,
   pun, quiet pleasure, quip, rag, razz, react, recreation,
   recreational, relish, rib, rib tickler, ridicule, riot, run,
   satisfaction, scintillate, scoff at, scream, self-gratification,
   self-indulgence, sensual pleasure, sensuous pleasure, serve,
   sexual pleasure, sick joke, sidesplitter, sight gag, sparkle,
   sport, story, sweetness of life, take, teasing, titillating,
   titillation, titillative, utter a mot, visual joke, voluptuousness,
   well-being, wheeze, wisecrack, work, wow, yarn, zest



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

FUN
       Flow Update Notification [message] (LFAP)


The Free On-line Dictionary of Computing (30 December 2018):

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).