Gloomily and silently bad-tempered.
Gloomily and silently bad-tempered; showing a heavy, brooding ill humor and a refusal to be sociable or cheerful.
Sullen is the heavy, weighted cousin of anger and resentment: the sullen person goes quiet out of a slow-burning, faintly embittered anger, fending off friendly overtures through silence and a darkened, downcast manner rather than through speech. It differs from open anger by being inward and mute, and from sadness or melancholy by carrying a charge of grievance rather than pure sorrow — though it shades toward gloom and broods like a dark sky. Against petulant it is graver and more enduring: petulant flares in childish, snippy outbursts, while sullen settles and stews. Against sulky it is more adult, more leaden, and less performative — sulky pouting often half-invites notice, whereas sullenness simply shuts the door. It is worth contrasting the Tagalog tampo, an affectionate sulking withdrawal that quietly hopes to be coaxed back; sullenness, by comparison, offers no such warmth and seeks no reconciliation.
1570s, an alteration of Middle English solein, soleyn "unique, singular, strange; solitary, lone, unmarried" (late 14c.), from Anglo-French solein, on the pattern of Old French solain "lonely," from soul "single," from Latin solus "by oneself, alone." The sense shifted from "solitary" to "morose" — i.e. "remaining alone through ill-humor" — beginning in the late 14c., after which the older senses faded.
The word's center of gravity moved from "alone, solitary" to "morose, ill-humored" as English speakers fused the idea of withdrawal with that of resentful gloom; by the 1570s the modern spelling and the "morose" sense were established, and the literal "solitary" senses dropped away. The meaning has been stable since, denoting a silent, brooding bad temper.