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]

Moby Thesaurus II by Grady Ward, 1.0:

203 Moby Thesaurus words for "inanity":
   absence of mind, absurdity, agnosticism, aimlessness, aridity,
   asininity, barrenness, battiness, big deal, blank, blank mind,
   blankmindedness, blankness, bloodlessness, bootlessness,
   brainlessness, buffoonery, callowness, calm of mind,
   characterlessness, clean slate, clownishness, colorlessness,
   counterproductiveness, counterproductivity, crackpottedness,
   crankiness, craziness, daffiness, dead letter, deadness,
   desipience, dismalness, dottiness, dragginess, dreariness, dryness,
   dullness, dustiness, eccentricity, effeteness, emptiness,
   emptiness of mind, empty sound, empty space, empty-headedness,
   etiolation, failure, fallow mind, fatuity, fatuousness,
   fecklessness, flatness, flavorlessness, flimsiness, folly, foolery,
   foolheadedness, foolishness, frivolity, frivolousness,
   fruitlessness, futility, giddiness, goofiness, greenhornism,
   greenness, heaviness, hiatus of learning, hollowness, idiocy,
   idleness, ignorance, ignorantism, ignorantness, imbecility,
   impotence, ineffectiveness, ineffectuality, ineffectualness,
   inefficaciousness, inefficacy, ineptitude, inexcitability,
   inexperience, innocence, insanity, insignificance, insipidity,
   insipidness, invalidity, jejuneness, jejunity, know-nothingism,
   knowledge-gap, lack of information, leadenness, levity,
   lifelessness, lightness, lowness of spirit, lunacy, madness,
   meaninglessness, mental blankness, mental void, mere noise,
   mildness, mindlessness, nescience, niaiserie, nirvana, noise,
   nonsensicality, nothing, nothingness, nugacity, nullity, nuttiness,
   oblivion, obscurantism, otiosity, paleness, pallor, passivity,
   phatic communion, pointlessness, pokiness, ponderousness,
   preposterousness, profitlessness, purposelessness, queerness,
   quietism, rat race, rawness, saplessness, sappiness, savorlessness,
   screwiness, senselessness, shallowness, silliness, simpleness,
   simplicity, slenderness, slightness, slowness, solemnity,
   spiritlessness, staleness, sterility, stiffness, stodginess,
   stuffiness, stupidity, superficiality, tabula rasa, tastelessness,
   tediousness, the absurd, thinness, thoughtfreeness,
   thoughtlessness, tranquillity, triflingness, triteness, triviality,
   trivialness, unacquaintance, unfamiliarity, unintelligence,
   uninterestingness, unknowing, unknowingness, unliveliness,
   unmeaningness, unproductiveness, unprofitability, unprofitableness,
   unripeness, unsavoriness, unsignificancy, uselessness, vacancy,
   vacuity, vacuousness, vacuum, valuelessness, vanity, vapidity,
   vapidness, vicious circle, void, wackiness, weakness, weirdness,
   wishy-washiness, witlessness, woodenness, worthlessness, zaniness,
   zanyism