Origin and history of sulk

"quietly sullen, silently resentful, moody and aloof, disposed to repel friendly advances," 1744, a word of uncertain origin. There is no record of it in Middle English. Connection has been suggested to the obsolete adjective sulke "hard to sell" (1630s) and to Old English asolcen "idle, lazy, slow."

This is a past-participle adjective from aseolcan "become sluggish, be weak or idle" (related to besylcan "be languid"), from Proto-Germanic *seklan (source also of Middle High German selken "to drop, fall").

But words of meaning similar to sulky often are held to be imitative (compare miff, mope, pout, boudoir). Related: Sulkily; sulkiness.