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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WordNet (r) 3.0 (2006):

sullenly
    adv 1: in a sullen manner; "he sat in his chair dourly" [syn:
           dourly, sullenly, glumly]