The Collaborative International Dictionary of English v.0.48:

Sullen \Sul"len\, a. [OE. solein, solain, lonely, sullen;
   through Old French fr. (assumed) LL. solanus solitary, fr. L.
   solus alone. See Sole, a.]
   1. Lonely; solitary; desolate. [Obs.] --Wyclif (Job iii. 14).
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   2. Gloomy; dismal; foreboding. --Milton.
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            Solemn hymns so sullen dirges change. --Shak.
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   3. Mischievous; malignant; unpropitious.
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            Such sullen planets at my birth did shine. --Dryden.
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   4. Gloomily angry and silent; cross; sour; affected with ill
      humor; morose.
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            And sullen I forsook the imperfect feast. --Prior.
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   5. Obstinate; intractable.
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            Things are as sullen as we are.       --Tillotson.
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   6. Heavy; dull; sluggish. "The larger stream was placid, and
      even sullen, in its course." --Sir W. Scott.
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   Syn: Sulky; sour; cross; ill-natured; morose; peevish;
        fretful; ill-humored; petulant; gloomy; malign;
        intractable.

   Usage: Sullen, Sulky. Both sullen and sulky show
          themselves in the demeanor. Sullenness seems to be an
          habitual sulkiness, and sulkiness a temporary
          sullenness. The former may be an innate disposition;
          the latter, a disposition occasioned by recent injury.
          Thus we are in a sullen mood, and in a sulky fit.
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                No cheerful breeze this sullen region knows;
                The dreaded east is all the wind that blows.
                                                  --Pope.
          [1913 Webster] -- Sul"len*ly, adv. --
          Sul"len*ness, n.
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The Collaborative International Dictionary of English v.0.48:

Sullen \Sul"len\, n.
   1. One who is solitary, or lives alone; a hermit. [Obs.]
      --Piers Plowman.
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   2. pl. Sullen feelings or manners; sulks; moroseness; as, to
      have the sullens. [Obs.] --Shak.
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The Collaborative International Dictionary of English v.0.48:

Sullen \Sul"len\, v. t.
   To make sullen or sluggish. [Obs.]
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         Sullens the whole body with . . . laziness. --Feltham.
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WordNet (r) 3.0 (2006):

sullen
    adj 1: showing a brooding ill humor; "a dark scowl"; "the
           proverbially dour New England Puritan"; "a glum, hopeless
           shrug"; "he sat in moody silence"; "a morose and
           unsociable manner"; "a saturnine, almost misanthropic
           young genius"- Bruce Bliven; "a sour temper"; "a sullen
           crowd" [syn: dark, dour, glowering, glum,
           moody, morose, saturnine, sour, sullen]
    2: darkened by clouds; "a heavy sky" [syn: heavy, lowering,
       sullen, threatening]

Moby Thesaurus II by Grady Ward, 1.0:

154 Moby Thesaurus words for "sullen":
   antisocial, at odds, autistic, averse, bad-tempered, balking,
   balky, bashful, beetle-browed, bigoted, black, black-browed,
   brooding, broody, bulldogged, bulletheaded, bullheaded,
   cantankerous, case-hardened, chapfallen, choleric, churlish, close,
   contrary, crabbed, crabby, cranky, crestfallen, cross,
   cross-grained, crotchety, crusty, cursory, cynical, dark, dejected,
   depressed, depressing, differing, difficult, disagreeing,
   disinclined, dismal, disobedient, dissociable, dogged, dogmatic,
   dour, dreary, dumpish, dyspeptic, fanatic, forced, fractious,
   fretful, froward, frowning, funereal, gloomy, glowering, glum,
   grim, grum, grumpy, hardheaded, headstrong, hostile, ill-humored,
   ill-natured, ill-tempered, incompatible, indisposed, indocile,
   insociable, intolerant, involuntary, irascible, long-faced,
   lowering, lugubrious, malevolent, malicious, malign, mean,
   melancholy, moodish, moody, mopey, moping, mopish, morose, mulish,
   mumpish, mutinous, nongregarious, obstinate, opinionated, opposed,
   ornery, out of humor, out of sorts, overzealous, peevish,
   perfunctory, persevering, pertinacious, perverse, pessimistic,
   petulant, pigheaded, pouting, recalcitrant, refractory, resistant,
   restive, runty, saturnine, scowling, self-contained,
   self-sufficient, self-willed, set, snug, socially incompatible,
   sour, splenetic, stiff-necked, strong-willed, strongheaded,
   stubborn, stuffy, sulking, sulky, surly, temperamental, tenacious,
   tenebrose, tenebrous, ugly, unclubbable, uncommunicative,
   uncompanionable, uncongenial, unconsenting, uncooperative,
   unfriendly, ungenial, unregenerate, unsociable, unsocial,
   unwilling, wayward, willful, wrongheaded