The Collaborative International Dictionary of English v.0.48:

Ponderousness \Pon"der*ous*ness\, n.
   The quality or state of being ponderous; ponderosity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ponderousness
    n 1: an oppressive quality that is laborious and solemn and
         lacks grace or fluency; "a book so serious that it
         sometimes subsided into ponderousness"; "his lectures tend
         to heaviness and repetition" [syn: ponderousness,
         heaviness]
    2: the property of being large in mass [syn: heft,
       heftiness, massiveness, ponderousness, ponderosity]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "ponderousness":
   Gothicism, aridity, avoirdupois, awkwardness, bad taste, barbarism,
   barbarousness, barrenness, beef, beefiness, bloodlessness,
   bombasticness, boorishness, bulkiness, bumblingness, cacology,
   cacophony, characterlessness, clownishness, clumsiness, coarseness,
   colorlessness, crudeness, cumbersomeness, cumbrousness, deadness,
   deadweight, dismalness, dragginess, dreariness, dryness, dullness,
   dustiness, dysphemism, effeteness, emptiness, etiolation, fatness,
   flatness, gawkiness, gawkishness, gracelessness, gravity,
   gross weight, grossness, ham-handedness, handful of thumbs,
   harshness, heaviness, heavy-handedness, heft, heftiness,
   hollowness, hulkiness, ill-balanced sentences, impracticality,
   impropriety, impurity, inanity, inconcinnity, inconvenience,
   incorrectness, indecorousness, inelegance, inelegancy,
   inexcitability, infelicity, insipidity, insipidness, jejunity,
   lack of finish, lack of polish, leadenness, left-handedness,
   lifelessness, liveweight, loutishness, lowness of spirit,
   lubberliness, lumpishness, maladroitness, massiveness, neat weight,
   net, net weight, oafishness, overbalance, overweight, paleness,
   pallor, pointlessness, pokiness, pompousness, ponderability,
   ponderosity, poor diction, poundage, roughness, rudeness,
   sesquipedalianism, sesquipedality, slipshod construction, slowness,
   solemnity, spiritlessness, sterility, stiffness, stiltedness,
   stodginess, stuffiness, superficiality, tastelessness, tediousness,
   tonnage, turgidity, uncouthness, underweight, uneuphoniousness,
   ungainliness, ungracefulness, unhandiness, uninterestingness,
   unliveliness, unmanageability, unrefinement, unseemliness,
   unwieldiness, vapidity, vapidness, vulgarism, vulgarity, weight,
   weightiness, woodenness