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):

frivolousness
    n 1: the trait of being frivolous; not serious or sensible [syn:
         frivolity, frivolousness] [ant: earnestness,
         serious-mindedness, seriousness, sincerity]