1781, back-formation of sulky (adj.).
"sullen," 1744, probably from O.E. asolcen "idle, lazy, slow," from pp. of aseolcan "become sluggish, be weak or idle" (related to besylcan "be languid"), from P.Gmc. *seklanan (cf. M.H.G. selken "to drop, fall").
My six-year-old has perfected the art of sulking when told he cannot have or do something he wants.