The Collaborative International Dictionary of English v.0.48:

massive \mass"ive\ (m[.a]s"[i^]v), a. [F. massif.]
   1. Forming, or consisting of, a large mass; compacted;
      weighty; heavy; massy. "Massive armor." --Dr. H. More.
      [1913 Webster]

   2. (Min.) In mass; not necessarily without a crystalline
      structure, but having no regular form; as, a mineral
      occurs massive.
      [1913 Webster]

   Massive rock (Geol.), a compact crystalline rock not
      distinctly schistose, as granite; also, with some authors,
      an eruptive rock.
      [1913 Webster]

WordNet (r) 3.0 (2006):

massive
    adj 1: imposing in size or bulk or solidity; "massive oak
           doors"; "Moore's massive sculptures"; "the monolithic
           proportions of Stalinist architecture"; "a monumental
           scale" [syn: massive, monolithic, monumental]
    2: being the same substance throughout; "massive silver"
    3: imposing in scale or scope or degree or power; "massive
       retaliatory power"; "a massive increase in oil prices";
       "massive changes"
    4: consisting of great mass; containing a great quantity of
       matter; "Earth is the most massive of the terrestrial
       planets"

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "massive":
   Brobdingnagian, Cyclopean, Gargantuan, adamantine, amplitudinous,
   astronomical, awesome, awkward, big, boundless, broad-bodied,
   bulky, bullnecked, burdensome, close, close-knit, close-textured,
   close-woven, clumsy, coarse, colossal, compact, compacted,
   compressed, concentrated, concrete, condensed, congested,
   consolidated, corpulent, cosmic, cracking, crammed, crammed full,
   crass, crowded, cumbersome, cumbrous, dense, durable, elephantine,
   enduring, enormous, extensive, fantastic, fat, firm, full,
   full-bodied, galactic, gargantuan, gigantic, gluey, grand, gross,
   hard, heavy, heavyset, heavyweight, hefty, huge, hulking, hulky,
   humongous, immeasurable, immense, impenetrable, impermeable,
   incumbent, infinite, jam-packed, jammed, jumbo, king-size, large,
   lasting, leaden, lubberly, lumbering, lumpish, lumpy, mammoth,
   massy, mighty, monster, monstrous, monumental, mortal, mountainous,
   nonporous, onerous, oppressive, outsize, overgrown, oversized,
   overweight, packed, ponderous, prodigious, rugged, serried,
   sizable, solid, sound, spacious, stable, steady, stout, strong,
   stupendous, sturdy, substantial, superincumbent, thick,
   thick-bodied, thick-growing, thickset, three-dimensional, titanic,
   tough, towering, tremendous, unwieldy, unyielding, vast, viscid,
   viscose, viscous, voluminous, walloping, weighty, well-built,
   well-constructed, well-founded, well-grounded, well-made,
   whopping