The Collaborative International Dictionary of English v.0.48:

Frivolous \Friv"o*lous\, a. [L. frivolus; prob. akin to friare
   to rub, crumble, E. friable: cf. F. frivole.]
   [1913 Webster]
   1. Of little weight or importance; not worth notice; slight;
      as, a frivolous argument. --Swift.
      [1913 Webster]

   2. Given to trifling; marked with unbecoming levity; silly;
      interested especially in trifling matters.
      [1913 Webster]

            His personal tastes were low and frivolous.
                                                  --Macaulay.

   Syn: Trifling; trivial; slight; petty; worthless. --
        Friv"o*lous*ly, adv. -- Friv"o*lous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

frivolous
    adj 1: not serious in content or attitude or behavior; "a
           frivolous novel"; "a frivolous remark"; "a frivolous
           young woman" [ant: serious]

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "frivolous":
   aimless, airy, amorphous, asinine, birdbrained, birdwitted,
   brainless, buxom, capricious, casual, catchpenny, childish,
   desultory, disarticulated, discontinuous, disjunct, disordered,
   dispersed, disproportionate, dizzy, empty, empty-headed, erratic,
   fatuous, featherbrained, featherheaded, fitful, flighty, flimsy,
   flip, flippant, fluffy, fluttery, foolish, formless, fribble,
   fribbling, frothy, futile, gaga, giddy, giddy-brained,
   giddy-headed, giddy-pated, giddy-witted, gleeful, gratuitous,
   haphazard, harebrain, harebrained, hilarious, hit-or-miss, idle,
   immethodical, inane, inchoate, incoherent, inconsequential,
   indiscriminate, insignificant, irregular, irresponsible, jocular,
   jocund, jolly, jovial, joyful, joyous, laughter-loving, light,
   meaningless, merry, minor, mirth-loving, mirthful, misshapen,
   niggling, nonsymmetrical, nonsystematic, nonuniform, nugacious,
   nugatory, orderless, otiose, paltry, peripheral, petty, planless,
   playful, promiscuous, puerile, random, rattlebrained, rattleheaded,
   rattlepated, rejoicing, risible, scatterbrained, scramblebrained,
   senseless, shallow, shallow-headed, shallow-minded, shallow-pated,
   shallow-witted, shapeless, shatterbrained, silly, slender, slight,
   small-time, spasmodic, sporadic, straggling, straggly, superficial,
   systemless, thoughtless, trifling, trite, trivial, two-bit,
   twopenny, unarranged, unclassified, undirected, ungraded,
   unimportant, unjoined, unmethodical, unordered, unorganized,
   unprofound, unsorted, unsymmetrical, unsystematic, ununiform,
   vacuous, vague, vain, vapid, volatile, wandering, windy, witless,
   worthless