[syn: dense, dim, dull, dumb, obtuse, slow]
The Collaborative International Dictionary of English v.0.48:
Dense \Dense\, a. [L. densus; akin to Gr. ? thick with hair or
   leaves: cf. F. dense.]
   1. Having the constituent parts massed or crowded together;
      close; compact; thick; containing much matter in a small
      space; heavy; opaque; as, a dense crowd; a dense forest; a
      dense fog.
      [1913 Webster]
            All sorts of bodies, firm and fluid, dense and rare.
                                                  --Ray.
      [1913 Webster]
            To replace the cloudy barrier dense.  --Cowper.
      [1913 Webster]
   2. Stupid; gross; crass; as, dense ignorance.
      [1913 Webster]
WordNet (r) 3.0 (2006):
dense
    adj 1: permitting little if any light to pass through because of
           denseness of matter; "dense smoke"; "heavy fog";
           "impenetrable gloom" [syn: dense, heavy,
           impenetrable]
    2: hard to pass through because of dense growth; "dense
       vegetation"; "thick woods" [syn: dense, thick]
    3: having high relative density or specific gravity; "dense as
       lead"
    4: slow to learn or understand; lacking intellectual acuity; "so
       dense he never understands anything I say to him"; "never met
       anyone quite so dim"; "although dull at classical learning,
       at mathematics he was uncommonly quick"- Thackeray; "dumb
       officials make some really dumb decisions"; "he was either
       normally stupid or being deliberately obtuse"; "worked with
       the slow students" [syn: dense, dim, dull, dumb,
       obtuse, slow]
Moby Thesaurus II by Grady Ward, 1.0:
161 Moby Thesaurus words for "dense":
   Boeotian, adamantine, asinine, beef-brained, beef-witted,
   blockheaded, blockish, bony, bovine, bristling, broad-bodied,
   bulky, bullnecked, cement, cemental, chumpish, cloddish, close,
   close-knit, close-textured, close-woven, coarse, compact,
   compacted, compressed, concentrated, concrete, condensed,
   congested, consolidated, corneous, corpulent, cowish, crammed,
   crammed full, crass, crawling, crowded, diamondlike, dim,
   dim-witted, doltish, dull, dullard, dumb, duncical, duncish,
   durable, dure, enduring, exuberant, fat, fatheaded, firm,
   flintlike, flinty, flourishing, foolish, full, full-bodied, gluey,
   granitelike, granitic, gross, hard, hard as nails, hardhearted,
   heaped, heavy, heavyset, horny, impassable, impassive,
   impenetrable, impermeable, ineducable, iron-hard, ironlike,
   jam-packed, jammed, jungled, jungly, klutzy, lapideous, lasting,
   lethargic, lithoid, lithoidal, lumpish, lush, luxuriant, marble,
   marblelike, massed, massive, nonporous, numskulled, oafish,
   obdurate, obtuse, opaque, osseous, overgrown, overrun, packed,
   phlegmatic, piled, populous, rank, resistant, resistive, riotous,
   rocklike, rocky, rugged, serried, slow, slow-witted, sluggish,
   solid, sottish, sound, stable, steady, steellike, steely, stolid,
   stonelike, stony, stout, strong, stupid, sturdy, substantial,
   swarming, teeming, thick, thick-bodied, thick-growing,
   thick-witted, thickheaded, thickset, three-dimensional, tight,
   torpid, tough, unteachable, unweeded, unyielding, viscid, viscose,
   viscous, weed-choked, weed-ridden, weedy, well-built,
   well-constructed, well-founded, well-grounded, well-made,
   wrongheaded