—adj.
1 morose, resentful, sulky, unforgiving, unsociable.
2 a (of a thing) slow-moving. b dismal, melancholy (a sullen sky).
—n. (in pl., usu. prec. by the) archaic a sullen frame of mind; depression.
Derivatives:
sullenly adv. sullenness n.
Etymology: 16th-c. alt. of ME solein f. AF f. sol SOLE(3)
Useful english dictionary. 2012.