The Collaborative International Dictionary of English v.0.48:

Ponderous \Pon"der*ous\, a. [L. ponderosus, from pondus, -eris,
   a weight: cf. F. pond['e]reux. See Ponder.]
   1. Very heavy; weighty; as, a ponderous shield; a ponderous
      load; the ponderous elephant.
      [1913 Webster]

            The sepulcher . . .
            Hath oped his ponderous and marble jaws. --Shak.
      [1913 Webster]

   2. Important; momentous; forcible. "Your more ponderous and
      settled project." --Shak.
      [1913 Webster]

   3. Heavy; dull; wanting; lightless or spirit; as, a ponderous
      style; a ponderous joke.
      [1913 Webster]

   Ponderous spar (Min.), heavy spar, or barytes. See
      Barite.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ponderous
    adj 1: slow and laborious because of weight; "the heavy tread of
           tired troops"; "moved with a lumbering sag-bellied trot";
           "ponderous prehistoric beasts"; "a ponderous yawn" [syn:
           heavy, lumbering, ponderous]
    2: having great mass and weight and unwieldiness; "a ponderous
       stone"; "a ponderous burden"; "ponderous weapons"
    3: labored and dull; "a ponderous speech"

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "ponderous":
   Latinate, all thumbs, arid, awkward, barren, big, blah, blank,
   bloodless, blunderheaded, blundering, bombastic, boorish, boring,
   bulky, bumbling, bungling, butterfingered, careless, characterless,
   clownish, clumsy, clumsy-fisted, cold, colorless, contrary,
   cramped, crosswise, cumbersome, cumbrous, dead, dismal, draggy,
   drearisome, dreary, dry, dryasdust, dull, dusty, effete,
   elephantine, empty, etiolated, fade, fat, fingers all thumbs, flat,
   forced, formal, fumbling, gauche, gawkish, gawky, graceless,
   grandiloquent, guinde, halting, ham-fisted, ham-handed, heavy,
   heavy-handed, heavyweight, hefty, ho-hum, hollow, huge, hulking,
   hulky, impractical, inane, inconvenient, inelegant, inexcitable,
   inflated, inkhorn, insipid, jejune, labored, laborious, large,
   leaden, left-hand, left-handed, lifeless, long-winded, loutish,
   low-spirited, lubberly, lumbering, lumpish, lumpy, maladroit,
   massive, massy, oafish, overdone, overweight, pale, pallid,
   pedestrian, perverse, plodding, pointless, poky, pompous, prolix,
   sesquipedalian, sloppy, slow, solemn, spiritless, sterile, stiff,
   stilted, stodgy, stuffy, superficial, tasteless, tedious, tiresome,
   turgid, uncouth, ungainly, ungraceful, unhandy, unlively,
   unmanageable, unwieldy, vapid, verbose, weighty, windy, wooden,
   wordy