The Collaborative International Dictionary of English v.0.48:

Sulkiness \Sulk"i*ness\, n. [For sulkenness, fr. AS. solcen
   slothful, remiss, in [=a]solcen, besolcen, properly p. p. of
   sealcan in [=a]sealcan to be weak or slothful; of uncertain
   origin.]
   The quality or state of being sulky; sullenness; moroseness;
   as, sulkiness of disposition.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sulkiness
    n 1: a mood or display of sullen aloofness or withdrawal;
         "stayed home in a sulk" [syn: sulk, sulkiness]
    2: a feeling of sulky resentment [syn: sulkiness,
       huffishness]
    3: a sullen moody resentful disposition [syn: sulkiness,
       sullenness, moroseness, sourness]