The Collaborative International Dictionary of English v.0.48:

Frivolity \Fri*vol"i*ty\, n.; pl. Frivolities. [Cg. F.
   frivolit['e]. See Frivolous.]
   The condition or quality of being frivolous; also, acts or
   habits of trifling; unbecoming levity of disposition.
   [1913 Webster]

WordNet (r) 3.0 (2006):

frivolity
    n 1: the trait of being frivolous; not serious or sensible [syn:
         frivolity, frivolousness] [ant: earnestness,
         serious-mindedness, seriousness, sincerity]
    2: something of little value or significance [syn: bagatelle,
       fluff, frippery, frivolity]
    3: acting like a clown or buffoon [syn: buffoonery,
       clowning, japery, frivolity, harlequinade, prank]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "frivolity":
   amusement, asininity, battiness, big deal, brainlessness,
   buffoonery, clownishness, crackpottedness, crankiness, craziness,
   daffiness, dallying, desipience, eccentricity, emptiness, fatuity,
   fatuousness, flightiness, flimsiness, flippancy, flirting, folly,
   foolery, foolheadedness, foolishness, frivolousness, fun, futility,
   game, giddiness, glee, gleefulness, goofiness, high glee,
   hilariousness, hilarity, idiocy, idleness, imbecility, inanity,
   ineptitude, insanity, jest, jocularity, jocundity, jolliness,
   jollity, joviality, joy, joyfulness, joyousness, laughter, levity,
   lightness, lunacy, madness, merriment, merriness, mindlessness,
   mirth, mirthfulness, niaiserie, nugacity, nuttiness, play,
   playfulness, prankishness, pranksomeness, queerness, roguishness,
   sappiness, screwiness, senselessness, shallowness, silliness,
   slenderness, slightness, sport, sportiveness, stupidity,
   superficiality, thoughtlessness, toying, trickery, trickiness,
   trifling, triflingness, triteness, triviality, trivialness,
   vacuity, vanity, vapidity, volatility, wackiness, waggery,
   waggishness, weirdness, witlessness, zaniness, zanyism