adj. & n.
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. sullenly adv. sullenness n.
[ 16th-c. alt. of ME solein f. AF f. sol SOLE(3) ]