The Collaborative International Dictionary of English v.0.48:

Leaden \Lead"en\, a.
   1. Made of lead; of the nature of lead; as, a leaden ball.
      [1913 Webster]

   2. Like lead in color, etc.; as, a leaden sky.
      [1913 Webster]

   3. Heavy; dull; sluggish. "Leaden slumber." --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

leaden
    adj 1: darkened with overcast; "a dark day"; "a dull sky"; "the
           sky was leaden and thick" [syn: dull, leaden]
    2: made heavy or weighted down with weariness; "his leaden
       arms"; "weighted eyelids" [syn: leaden, weighted]
    3: made of lead; "a leaden weight"
    4: (of movement) slow and laborious; "leaden steps" [syn:
       leaden, plodding]
    5: lacking lightness or liveliness; "heavy humor"; "a leaden
       conversation" [syn: heavy, leaden]

Moby Thesaurus II by Grady Ward, 1.0:

274 Moby Thesaurus words for "leaden":
   Latinate, Quaker-colored, abeyant, achromatic, achromic, acier,
   anemic, apathetic, arid, ashen, ashy, aureate, awkward, barren,
   benumbed, blah, bled white, bloodless, bombastic, bored, brass,
   brassy, brazen, bronze, bronzy, burdensome, cadaverous, canescent,
   cataleptic, catatonic, characterless, chloranemic, cinereous,
   cinerous, cold, colorless, copper, coppery, cramped, cumbrous,
   cupreous, cuprous, dapple, dapple-gray, dappled, dappled-gray,
   dark, dead, deadly pale, deathly pale, debilitated, dense, dim,
   dimmed, dingy, discolored, dismal, dopey, dormant, dove-colored,
   dove-gray, draggy, drearisome, dreary, droopy, drugged, dry,
   dryasdust, dull, dusty, effete, elephantine, empty, enervated,
   etiolated, exanimate, exsanguinated, exsanguine, exsanguineous,
   fade, faded, faint, fallow, fat, ferrous, ferruginous, flat,
   forced, formal, foul, ghastly, gilt, glaucescent, glaucous, gloomy,
   glowering, gold, gold-filled, gold-plated, golden, gray,
   gray-black, gray-brown, gray-colored, gray-drab, gray-green,
   gray-spotted, gray-toned, gray-white, grayed, grayish, grey,
   griseous, grizzle, grizzled, grizzly, groggy, guinde, haggard,
   halting, heavy, heavyweight, hebetudinous, hefty, ho-hum, hollow,
   hueless, hypochromic, in abeyance, in suspense, inactive, inane,
   inanimate, inert, inexcitable, inkhorn, insipid, iron, iron-gray,
   ironlike, jaded, jejune, labored, lackadaisical, lackluster,
   languid, languorous, latent, lead, lead-gray, lethargic, lifeless,
   listless, livid, logy, low-spirited, lowering, lumbering, lumpish,
   lurid, lusterless, massive, massy, mat, mealy, mercurial,
   mercurous, moribund, mouse-colored, mouse-gray, mousy, muddy,
   neutral, nickel, nickelic, nickeline, numb, numbing, onerous,
   oppressive, overweight, pale, pale as death, pale-faced, pallid,
   passive, pasty, pearl, pearl-gray, pearly, pedestrian, pewter,
   pewtery, phlegmatic, plodding, pointless, poky, pompous, ponderous,
   pooped, quicksilver, sad, sallow, sated, sedentary, sesquipedalian,
   sickly, silver, silver-gray, silver-plated, silvered, silvery,
   slack, slate-colored, slaty, sleeping, sleepy, slow, sluggish,
   slumbering, smoke-gray, smoky, smoldering, sober, solemn, somber,
   somnolent, soporific, spiritless, stagnant, stagnating, standing,
   static, steel, steel-gray, steely, sterile, stiff, stilted, stodgy,
   stone-colored, stuffy, stultified, sullen, superficial, supine,
   suspended, tallow-faced, tame, tasteless, taupe, tedious, tin,
   tinny, toneless, torpid, turgid, unaroused, uncolored, unlively,
   unwieldy, vapid, vegetable, vegetative, wan, washed-out, waxen,
   weak, weary, weighty, whey-faced, white, wooden, world-weary