Sullen means silently bad-tempered and resentful. The mood that will not explain itself and will not go away.
The silence is what defines it, since a sullen person refuses to engage rather than shouting. It suits teenagers, crowds and weather equally, and a sullen sky is a heavy grey one that has not yet rained. It is a shade darker than sulky, which is briefer and more childish. The adverb sullenly usually attaches to a verb of speaking or moving, and the noun is sullenness.
Latin
sōlus — by oneself alone
Old French
sole — single, sole, alone
Anglo-Norman
soleyn — alone
Middle English
soleyn
English, today
sullen
From Middle English soleyn, from Anglo-Norman soleyn (“alone”), from Old French sole (“single, sole, alone”), from Latin sōlus (“by oneself alone”). The change in meaning from "single" to morose occurred in Middle English.
Etymology from Wiktionary, CC BY-SA