The Collaborative International Dictionary of English v.0.48:

Inanity \In*an"i*ty\, n.; pl. Inanities. [L. inanitas, fr.
   inanis empty: cf. F. inanit['e]. See Inane.]
   [1913 Webster]
   1. Inanition; void space; vacuity; emptiness.
      [1913 Webster]

   2. Lack of seriousness; aimlessness; frivolity.
      [1913 Webster]

   3. An inane, useless thing or pursuit; a vanity; a silly
      object; -- chiefly in pl.; as, the inanities of the world.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inanity
    n 1: total lack of meaning or ideas [syn: inanity,
         senselessness, mindlessness, vacuity,
         pointlessness]