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]